Efficient coupling of nanosecond laser pulses with the cluster medium: Generation of hydrogen-like [C](5+) atomic ions.

Research Article Received: 10 September 2014

Revised: 28 October 2014

Accepted: 28 October 2014

Published online in Wiley Online Library

Rapid Commun. Mass Spectrom. 2015, 29, 45–53 (wileyonlinelibrary.com) DOI: 10.1002/rcm.7085

Efficient coupling of nanosecond laser pulses with the cluster medium: Generation of hydrogen-like [C]5+ atomic ions Pramod Sharma*, Soumitra Das and Rajesh K. Vatsa* Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India RATIONALE: Clusters exhibit diverse photochemical behavior as a function of laser parameters, i.e. wavelength, pulse duration and intensity. One such aspect of cluster photochemistry is the generation of energetic multiply charged atomic ions, upon efficient interaction of clusters with intense laser pulses. In the present work, mass spectrometric investigations have been carried out on clusters of tetrahydrofuran (THF, C4H8O) – a saturated cyclic ether – subjected to nanosecond laser pulse (spanning from UV to IR wavelength range) with the aim of shedding light on the complex mechanism of laser-cluster interactions, which is still ambiguous. METHODS: THF clusters, generated via supersonic expansion of room-temperature THF vapours seeded in argon, were subjected to gigawatt intensity laser pulses (355, 532 and 1064 nm) obtained from a nanosecond Nd:YAG laser. The ions generated upon laser-cluster interaction were characterized using a time-of-flight mass spectrometer. RESULTS: At 355 nm, THF clusters exhibit the usual multiphoton dissociation/ionization behavior while, at 532 nm, observation of multiply charged atomic ions of carbon (up to [C]4+) and oxygen (up to [O]3+) was ascribed to Coulomb explosion of THF clusters. For studies carried out at 1064 nm, multiply charged atomic ions of carbon up to [C]5+ having an ionization energy of ~392 eV were observed, at a laser intensity of 1010 W/cm2. CONCLUSIONS: The observation of [C]5+ atomic ions signifies efficient coupling of the laser energy with the cluster medium, using a nanosecond laser pulse. The results have been rationalized on the basis of a three-stage cluster ionization mechanism, suggesting the crucial role of the threshold laser intensity for initiating ionization within the cluster and generation of optimum charge centers for efficient extraction of energy from the laser pulse. Copyright © 2014 John Wiley & Sons, Ltd.

Clusters, defined as aggregates of atoms/molecules, are considered as facile targets for understanding different aspects of laser-matter interaction. Over the last few decades a large number of experimental and theoretical studies have been carried out to understand different properties of clusters, i.e. structure, stability, reactivity, etc., as a function of their size, to correlate the evolution of matter as it transforms from the atomic/molecular level to the bulk phase.[1–7] Because of the inherent property of clusters to provide bulk-like number density of atoms/molecules in the gas phase, they interact very efficiently with laser pulses compared with their isolated monomer counterparts. The energy extracted from the laser pulse is largely confined to the isolated cluster which is devoid of extrinsic energy dissipation channels, and the absorbed energy within the excited cluster is released by several intra-cluster processes, i.e. ionization, fragmentation, fission, Coulomb explosion, etc.[8] For cluster media subjected to intense short laser pulses (>1014 W/cm2), energy absorption efficiency >90% has been reported,[9] resulting in the generation of highly charged ([Xe]m+, m ≤40) energetic atomic ions (up to 1 MeV),[10,11]

Rapid Commun. Mass Spectrom. 2015, 29, 45–53

Copyright © 2014 John Wiley & Sons, Ltd.

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* Correspondence to: P. Sharma and R. K. Vatsa, Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400 085, India. E-mail: [email protected]; [email protected]

energetic electrons,[12] X-rays,[13] and even neutrons.[14] These studies have been motivated by the desire to enhance the extent of laser-cluster interactions, with implications for the development of table-top accelerators.[15,16] Hitherto, almost all reports on atomic and molecular clusters dealing with generation of energetic multiply charged atomic ions have utilized ultra-short laser pulses having intensity >1014 W/cm2. Under such high laser intensity, field ionization – coupled with high ponderomotive energies (~ few hundred eV) – ensures that the majority of the cluster constituents undergo excessive ionization.[17] Subsequently, these highly charged clusters disintegrate under the influence of a large intra-cluster coulombic repulsive potential (Coulomb explosion), caused by the close proximity of charged species within the cluster, resulting in the generation of multiply charged atomic ions with high kinetic energy. Over the last few years a significant number of reports have dealt with Coulomb explosion studies carried out on atomic and molecular clusters, using nanosecond laser pulses having intensity of ~109 to 1011 W/cm2.[18–21] Initially, observation of Coulomb explosion phenomena leading to the generation of energetic multiply charged atomic ions under such conditions was considered to be unusual, due to the lower cross-section of multiphoton excitation processes, which are predominant under nanosecond laser intensity conditions. However, nanosecond laser induced Coulomb explosion studies on various cluster systems have been reported and different

P. Sharma, S. Das and R. K. Vatsa facets of laser-cluster interactions have been probed to understand the mechanism of nanosecond laser induced Coulomb explosion, i.e. laser wavelength, cluster size, effect of doping, etc.[22–24] Although the interaction of nanosecond laser pulses with clusters is less efficient than that of picoand femtosecond lasers, observation of multiply charged atomic ions at ~105 to 106 lower laser intensity is a significant outcome. Efforts are under way to optimize different experimental parameters for enhancing the efficiency of laser-cluster interactions under nanosecond laser conditions by varying the nature of the cluster constituents, doping clusters with species having varying ionization energies, etc. To this end, the photochemical behavior of tetrahydrofuran clusters has been investigated in the present study. Tetrahydrofuran (THF) is generally considered as the simplest molecular analogue for the deoxyribose building block of DNA. A number of experimental[25–27] and theoretical[28] studies have been carried out on THF, mainly to simulate the radiation damage caused to the biomolecules upon interaction with ionizing radiations. It is known that in biological media ionizing radiations liberate large numbers of low-energy secondary electrons. Consequently, a number of electron interaction[29–32] and dissociative electron attachment[33,34] studies have been carried out on THF. The ionization and ionic fragmentation process of THF have also been studied in the electron energy range of 6–150 eV using mass spectrometry, and the appearance energy of selected fragment ions has been determined.[35] In contrast, there have only been a few reports regarding the photochemistry of THF and its clusters. In the gas phase, photofragmentation/photoionization studies on THF have been carried out in the ultraviolet (UV) and vacuum ultraviolet (VUV) regions. In the VUV region, Homem et al. measured the photoabsorption cross-section and ionization quantum yield for THF,[36] while the threshold photoelectron spectrum of THF has been investigated over the photon energy range of 9–29 eV.[37] The photodissociation dynamics of THF were studied at 193.3 nm, using photofragmenttranslational spectroscopy and vacuum ultraviolet photoionization. In that study, five dissociation channels for THF were identified, which primarily originate from the ground state potential-energy surface, following rapid internal conversion from the photo-excited state.[38] Young et al. studied THF cluster anions [(THF)n] using photoelectron imaging, to understand electron solvation in THF. Based on these studies, for an increase in cluster size from n = 6 to 100, the vertical detachment energy (VDE) of the solvated electron clusters was found to increase from 1.96 to 2.71 eV, suggesting that larger THF clusters readily solvate the free electrons due to preexisting voids.[39] The present work investigates the photochemistry of THF clusters upon interaction with laser pulses of ~109 to 1010 W/cm2 intensity using different harmonics of a nanosecond Nd:YAG laser, to understand the photofragmentation/photoionization behavior as a function of laser wavelength and intensity.

supersonic expansion of room-temperature THF vapours seeded in helium/argon with different backup pressures varying from 1 to 5 bar. A pulsed valve (0.8 mm nozzle diameter and 300 μs pulse duration) was used for this purpose. The experimental results were found to be independent of the carrier gas and so all the experimental results presented here were obtained using argon carrier gas. The gas jet so produced was skimmed at a distance of 5 cm from the pulsed nozzle and entered the mass spectrometer. Here, in the acceleration region of the inhouse built dual-stage time-of-flight mass spectrometer located 17 cm downstream from the skimmer, the clusters were subjected to nanosecond laser pulses (i.e. 355, 532 and 1064 nm), from a pulsed Nd:YAG laser (Quanta System, Olona, Italy; 10 ns, GIANT G790-10). The ions formed upon laser-cluster interaction were accelerated and guided into a 125-cm field-free region using the double-focusing Wiley-McLaren assembly and detected using a channel electron multiplier (CEM) detector. Typical voltages applied to the repeller and extractor grids were 2830 V and 1200 V, respectively. The ion signal from the CEM detector was recorded on a digital storage oscilloscope (LeCroy, Wavesurfer-42MXs-B, 400 MHz; Teledyne LeCroy, Bangalore, India) and further processed on a computer. The mass resolution of the instrument is ~300.

RESULTS Figure 1 depicts a typical time-of-flight (TOF) mass spectrum obtained when pure THF clusters were irradiated with a 355 nm laser pulse of intensity 1.4 × 109 W/cm2. The major ions observed in the mass spectrum were assigned to [H]+, [CHn]+ (n = 0–3), [C2Hn]+ (n = 0–3), [CHO]+, [C3H3]+, [C2H3O]+, [C4H7O]+ and [(C4H8O)H]+. Cluster ions of the type [(C4H8O)nH]+ (n = 2–6) were also observed in the mass spectrum along with a [(C4H8O)3(H2O)H]+ ion, which can arise from intra-cluster reactions within the THF cluster or from the presence of residual moisture in the sample tube/mass spectrometer. Interestingly, no ions corresponding

EXPERIMENTAL

46

Details of the experimental setup have been described in our earlier publications and only a brief account is given here.[17,24] Neutral clusters of THF were generated via

wileyonlinelibrary.com/journal/rcm

Figure 1. Time-of-flight mass spectrum of THF clusters subjected to 355 nm laser of intensity ~1.4 × 109 W/cm2.



Tetrahydrofuran clusters irradiated with nanosecond laser pulse to a water-associated THF cluster complex were observed for the monomer and dimer of THF. In addition, no C4H8O+ molecular ion was observed in the mass spectrum. The absence of a molecular ion has been ascribed to the tendency of C4H8O+ to undergo unimolecular dissociation via loss of a hydrogen atom, leading to the formation of a cyclic C4H7O+ fragment ion.[26] The appearance energy for C4H7O+ is ~10.2 eV, which is energetically accessible upon multiphoton ionization of THF clusters at 355 nm.[35] Under identical experimental conditions, when the THF clusters were subjected to 532 nm laser pulses of intensity ~4.5 × 109 W/cm2, [CHO]+, [C3H3]+, [C4H7O]+ and [(C4H8O)2H]+ fragment ions were observed in the mass spectrum (Fig. 2). In addition, ion signals observed at m/z 1, 3, 4, 5.33, 6, 8, 12 and 16 were assigned to [H]+, [C]4+, [C]3+, [O]3+, [C]2+, [O]2+, [C]+ and [O]+, respectively. The ion signal at m/z 4 could arise from either [C]3+ or [O]4+. However, as the ionization energy (I.E.) for the generation of [C]3+ from [C]2+ is lower than that for the generation of [O]4+ ions from [O]3+ ions (Table 1), it is assumed that the contribution from the [O]4+ ion to the peak at m/z 4 is negligible. No higher cluster ions were observed in the mass spectrum. Based on our previous studies carried out on different molecular clusters, we ascribe the generation of multiply charged atomic ions to the disintegration of excessively ionized THF clusters generated upon interaction with the 532 nm laser pulse, under the influence of Coulombic forces. In our experiments, all the ion signals arising from multiply charged atomic ions exhibited significant peak broadening in the TOF mass spectra, suggesting the release of a large amount of kinetic energy during the disassembly of a highly charged cluster under the influence of Coulombic repulsion (Fig. 3). As a result, upon disintegration of the cluster, for ions with the same m/z values, the ion signal constitutes a broad forward component and a sharp energyfocused backward component. Such a peak profile arises from isotropic ejection of ions upon disintegration of the multiply charged cluster. Consequently (for ions with same m/z value), those ions ejected along the TOF axis towards the detector arrive early in time (forward component), while the ions which are ejected towards the repeller plate (in the direction opposite

Table 1. Ionization energy required to produce given multiply charged atomic ions of carbon and oxygen[40] Charge state (n)

([C]n+) (eV)

([O]n+) (eV)

+1 +2 +3 +4 +5 +6 +7 +8

11.26 24.38 47.88 64.49 392 490

13.62 35.12 54.93 77.41 113.9 138.12 739.32 871.39

Figure 3. Time-of-flight mass spectrum depicting broad and split peaks obtained for different multiply charged atomic ions upon Coulomb explosion of THF clusters at 532 nm. to the detector) arrive late at the detector (backward component), as these ions are first decelerated, stopped and then directed back towards the detector. Thus, based on TOF equations, the kinetic energy gained by the ions from the Coulomb explosion process can be determined from the time separation between the forward and backward components of the ion peaks using the following equation: Ekin ðeVÞ ¼


(1)

where Δt (in nanoseconds) represents the time difference between the split mass peaks, E is the static electric field for ion extraction in V/cm, n is the charge, and m (in u) is the mass of the fragment ion.[41] Using the above equation, the kinetic energies of different atomic ions were calculated and are tabulated in Table 2. The comparison of results obtained upon interaction of THF clusters with 355 and 532 nm laser pulses suggests that 532 nm laser pulses interact more efficiently with THF clusters, resulting in the generation of multiply charged atomic ions having an ionization energy as high as 64.5 eV ([C]4+) (Table 1) and a kinetic energy of 100s of eV.



47

Figure 2. Time-of-flight mass spectrum of THF clusters subjected to 532 nm laser of intensity ~4.5 × 109 W/cm2.

9:65107 Δt2 n2 E2 8m

P. Sharma, S. Das and R. K. Vatsa Table 2. Kinetic energy of multiply charged atomic ions produced upon Coulomb explosion of THF clusters at 532 nm Kinetic energy of multiply charged atomic ions (eV) Laser intensity at 532 nm (W/cm2)

[C]+

[C]2+

[C]3+

[C]4+

[O]+

[O]2+

[O]3+

6 × 109

16

96

470

777

12

51

229

Understanding the laser-cluster interaction mechanism at 355 and 532 nm Based on the Keldysh parameter ’γ’, which broadly distinguishes the boundary between the multiphoton ionization (MPI) (γ > > 1) and the field ionization (γ > 1) at 355 and 532 nm. Here: rffiffiffiffiffiffiffiffiffi I:E: γ¼ (2) 2UP where I.E. is the ionization energy and Up is the ponderomotive energy.[42] Accordingly, laser power dependency studies were carried out for different cluster-generated ions at 355 and 532 nm. Figure 4 illustrates the logarithmic plots of ion signal vs peak laser intensity for representative ions obtained at 355 and 532 nm. Based on these plots, the rate-limiting multiphoton absorption process leading to the generation of different ions can be readily determined from the slope of the ln-ln plot. At 355 nm, a slope of ~2 was obtained for laser power dependency studies carried out on different ions, while, at

532 nm, a slope of ~3 was obtained. Thus, at 355 and 532 nm, two-photon and three-photon absorption processes are the rate-determining steps, respectively, and an intermediate excited state at ~7 eV plays a crucial role in the multiphoton ionization of THF clusters. The ionization energy of the THF cluster is expected to be ≤9.4 eV. Thus, based on laser power dependency studies, we can conclude that, at 355 nm, the THF cluster undergoes (2 + 1) multiphoton ionization and at 532 nm (3 + 1) the multiphoton ionization process leads to the generation of different ions. The observation of multiply charged atomic ions at 532 nm having an ionization energy as high as 64 eV ([C]4+), which requires concomitant absorption of a large number of photons (~28 photons) from the 532 nm laser pulse, cannot be explained merely on the basis of a multiphoton excitation process. However, multiphoton excitation of the THF clusters to an intermediate excited state at ~7 eV is expected to play a crucial role. In our previous studies,[23,24] the generation of multiply charged atomic ions under nanosecond laser pulse conditions was explained by a three-stage cluster ionization mechanism, i.e. "multiphoton ionization ignited-inverse bremsstrahlung heating–electron ionization",[18,19] which suggests that, under the influence of a nanosecond laser pulse, the clusters

48

Figure 4. Laser power dependency (ln-ln plot) for representative ions formed upon interaction of THF clusters with 355 nm – (a) C2H3O+, (b) C4H7O+ and 532 nm laser pulse – (c) C4+ and (d) C2H3O+.




Tetrahydrofuran clusters irradiated with nanosecond laser pulse undergo multiphoton ionization at the leading edge of the laser pulse. Subsequently, some of the liberated electrons get confined within the cluster (i.e. inner ionized electrons), and keep extracting energy from the remnant laser pulse via an inverse Bremsstrahlung (IBS) process during electron–ion and electron–neutral collisions, under the influence of the Coulomb field within the cluster. Once the electron energy exceeds the ionization energy of the species within the cluster, further ionization can occur by electron ionization resulting in augmentation of the charged state on the cluster. These sequences of events continue up to a stage where Coulombic repulsion overcomes the total cohesive energy of the cluster or until the end of the laser pulse. As a result, the multiply charged cluster explodes, resulting in the generation of energetic multiply charged atomic ions. The rate of energy extraction by the confined electrons via the IBS process from the laser pulse is given by Eqn. (3):[43,44] dE ¼ Up v dt

(3)

where v is the collision frequency of the inner ionized electrons and is of the order of ~1014 to 1015 Hz (i.e. order of laser frequency) and the ponderomotive energy (Up) is given by Eqn. (4) U p ¼ 9:331014 I W=cm2 λ2 ðμmÞ2

(4)

From Eqns. (3) and (4), it is obvious that the total energy gained by the inner ionized electrons, in the total time span starting from initial ionization till the disintegration of the cluster, is dictated by the product of the ponderomotive energy and the total number of effective electron-ion/neutral collisions. In addition, Eqn. (4) suggests that, as the wavelength (λ) increases, the ponderomotive energy (Up) of the electrons increases quadratically for a given laser intensity. Hence, a higher level of ionization is expected at longer laser wavelengths. This qualitatively explains our results obtained at 355 and 532 nm for the THF clusters and it is discussed further in subsequent sections. Thus, at 355 nm, THF clusters exhibit pure multiphoton ionization behavior, while, at 532 nm, they exhibit Coulomb explosion, induced by secondary ionization processes (as discussed above) resulting in the generation of multiply charged atomic ions. Interaction of THF clusters with 1064 nm laser wavelength


[O]6+, [C]4+, [O]5+, [C]3+/[O]4+, [O]3+, [C]2+, [O]2+ and [C]+ ions. Other ions observed are [C2H3]+, [CHO]+, [C3H3]+, [C3H5]+ and [C4H7O]+. In addition, ion signals corresponding to the argon carrier gas, i.e. [Ar]3+, [Ar]2+ and [Ar]+, were observed. As can be seen from the figure, the signals corresponding to multiply charged atomic ions are broad. However, under our experimental conditions, the kinetic energy of the multiply charged atomic ions could not be quantified at 1064 nm, as it was not possible to resolve the forward and backward components of the ion signal even at a higher ion extraction electric field. This indicates that the multiply charged atomic ions are highly energetic compared with the kinetic energy of multiply charged atomic ions measured at 532 nm. In addition, the overlapping nature of the ion signals for the multiply charged atomic ions and their close proximity also hindered our efforts to quantify the kinetic energy at 1064 nm. A significant finding of our present work is the observation of the multiply charged carbon ion [C]5+. So far in none of the laser-cluster interaction studies carried out using nanosecond laser pulses of intensity ~109 to 1010 W/cm2 has the removal of electrons from the inner core-shell been reported, to generate multiply charged atomic ions that require an ionization energy as high as 392 eV ([C]5+). In addition, on comparing Fig. 2 with Fig. 5, it can be concluded that the distribution of multiply charged atomic ions shifts towards higher charged states at 1064 nm. In all our previous studies carried out on different cluster systems at different laser wavelengths, the highest observed multiply charged state of carbon has been +4, although the



49

The above three-stage cluster ionization model hints that at longer laser wavelength a higher level of ionization is expected for THF clusters. Thus, studies on THF clusters were carried out at 1064 nm. No ion signal could be observed in the TOF mass spectrum at a laser intensity of ~109 W/cm2, under identical experimental conditions to those used for our studies at 355 and 532 nm. This suggests that the threshold laser intensity for multiphoton excitation of THF clusters leading to ionization is much higher, due to the lower multiphoton excitation cross-section of THF clusters at 1064 nm. Hence, further studies were carried out at 1010 W/cm2. Figures 5(a) and 5(b) depict the TOF mass spectrum of THF clusters subjected to 1064 nm laser pulses of intensity ~1.4 × 1010 W/cm2. The mass spectrum exhibits a rich fragmentation pattern, depicting the generation of [H]+, [C]5+,

Figure 5. Time-of-flight mass spectrum of THF cluster subjected to 1064 nm laser pulses of intensity ~1.4 × 1010 W/cm2. For clarity, the mass spectrum has been divided into (a) m/z 0–10 and (b) m/z 10–75 mass range. Inset in (b) depicts m/z 13–17.

P. Sharma, S. Das and R. K. Vatsa laser intensity had been varied from 109 W/cm2 to 1010 W/cm2 presuming that a higher laser intensity would facilitate the generation of more highly higher charged atomic ions. For comparison, Figs. 6(a) and 6(b) depict TOF mass spectra recorded for THF clusters subjected to 355 and 532 nm laser pulses of intensity ~7.4 × 109 W/cm2 and 1.1 × 1010 W/cm2, respectively. However, the increase in laser intensity only resulted in enhancement in the yield of ion signal, and no augmentation in the charge state of the multiply charged atomic ions was observed. Thus, on comparing the TOF mass spectrum recorded at 532 nm with that recorded at 1064 nm, it can be concluded that the 1064 nm laser pulses are comparatively more efficient in interacting with THF clusters. In addition, the relative ion yield of the higher multiply charged atomic ion is comparatively larger at 1064 nm, signifying the generation of higher charged state atomic ions at the expense of lower charged states.

DISCUSSION For THF clusters exposed to 1064 nm laser pulses of intensity ~1.4 × 1010 W/cm2, based on the Keldysh parameter γ, the interaction of THF clusters with 1064 nm laser wavelength lies in the MPI regime (γ ~47). Accordingly, laser power dependency studies were carried out for different ions generated at 1064 nm and a power dependency of ~8 was obtained (Fig. 7). Thus, even at 1010 W/cm2 in the case of the 1064 nm laser wavelength, the primary process for the generation of multiply charged atomic ions is initial

50

Figure 6. Time-of-flight mass spectra of THF clusters subjected to (a) 355 nm laser of intensity ~7.4 × 109 W/cm2 and (b) 532 nm laser of intensity ~1.1 × 1010 W/cm2.


multiphoton ionization of the THF clusters. This approach is similar to that used for the studies carried out on THF clusters at 355 and 532 nm, where a power dependency of ~2 and ~3 was obtained, respectively. Thus, based on laser power dependency studies, the multiphoton ionization probability of THF clusters at 1064 nm is expected to be significantly lower than at 355 and 532 nm. On the contrary, a higher level of ionization has been observed at 1064 nm. To account for this anomalous observation, based on the three-stage cluster ionization mechanism, the average kinetic energy gained by the inner ionized electron (which participates in secondary ionization of cluster constituents) has been calculated, assuming that once the ionization has been initiated within the cluster by multiphoton absorption at the leading edge of the laser pulse, the cluster survives for ~1 ns (hypothetical case) prior to disintegration under the influence of the repulsive Coulombic forces. The calculations suggest that the average energies gained by the inner ionized electron at 355, 532 and 1046 nm, at the laser intensities used in our experiment, are 13.9, 67.1 and 417.3 eV, respectively. Thus, the calculated average kinetic energy of the inner ionized electron at 532 and 1064 nm qualitatively explains the observation of the highest multiply charged atomic ions observed at both the laser wavelengths, i.e. [C]4+ (I.E. ~64.5 eV) at 532 nm and [C]5+ (I.E. ~392 eV) at 1064 nm. Therefore, for THF clusters generated under identical expansion conditions, upon initiation of multiphoton ionization along the leading edge of the laser pulse, the cluster survives for ≤1 ns, prior to undergoing Coulomb explosion. In contrast, for experiments carried out at higher laser intensities of 7.4 × 109 and 1.1 × 1010 W/cm2 at 355 and 532 nm, respectively, the calculations suggest that the inner ionized electron can gain kinetic energies of ~73.5 and 163.5 eV, at 355 and 532 nm, respectively. As a result, at 355 nm, multiply charged atomic ions up to [C]4+ and [O]3+ should have been observed at a laser intensity of 7.4 × 109 W/cm2. Similarly, at 532 nm, in addition to multiply charged atomic ions of carbon up to [C]4+, multiply charged atomic ions of oxygen up to [O]6+ should have been observed in the mass spectrum. However, at 355 nm, multiply charged atomic ions of carbon and oxygen were not observed, while, at 532 nm, the mass spectrum recorded at laser intensity 1.1 × 1010 W/cm2 was similar to that recorded at 4.5 × 109 W/cm2, with the highest observed charged states being [C]4+ and [O]3+, although an enhancement in the yield of ionic species was observed. These observations suggest that, at the higher laser intensities of 355 and 532 nm, the THF cluster disintegrates prematurely and the inner ionized electron is unable to acquire sufficient energy to cause further ionization. The above results can be interpreted on the basis of the multiphoton ionization probability (WMPI) of THF clusters, at different laser wavelengths. In general, the multiphoton ionization probability (WMPI = σ n In, where I is the laser intensity in photons/s/cm2) is conversely related to the number of photons (n) required in the ionization process, as the ionization cross-section (σ n, cm2nsn–1) decreases with the order (n) of the multiphoton ionization process.[45] Thus, based on laser power dependency studies, the multiphoton ionization probability of the THF clusters is expected to follow the order: 355WMPI > 532WMPI > > 1064WMPI. Consequently, at 355 and 532 nm, where two- and three-photon multiphoton absorption processes are responsible for initiation of ionization



Tetrahydrofuran clusters irradiated with nanosecond laser pulse

Figure 7. Laser power dependency (ln-ln plot) for representative ions formed upon interaction of THF clusters with 1064 nm laser pulse: (a) C+, (b) O+, (c) C2+, and (d) O2+. of the THF cluster, the ionization occurs along the leading edge of the laser pulse, once the threshold laser intensity has been achieved. As a result, few ionization centers are generated within the cluster, causing the cluster to expand in conjunction with energization of inner ionized electrons, and ultimately disintegrate. When the laser intensity of the 355 and 532 nm laser pulse is increased further, a comparatively large number of ionization centers are generated along the leading edge of the laser pulse – causing the cluster to expand rapidly. Therefore, the inner ionized electrons are unable to extract energy from the laser pulse to the extent expected on the basis of the calculations reported in Table 3. Thus, the atomic ions observed at a particular laser wavelength are independent of laser intensity. On the contrary, at 1064 nm as the threshold laser intensity for multiphoton ionization of the THF cluster is comparatively higher, ionization is expected to occur close to the peak of the laser pulse. As a result the inner ionized

electrons interact more efficiently with the laser pulse causing enhanced ionization of the cluster constituents. Thus, although the initial multiphoton ionization probability is higher at 355 and 532 nm, based on our studies a higher level of ionization is observed at 1064 nm. The present studies suggest that the energy extracted from the nanosecond laser pulse by the cluster medium is a complex function of the number of initial charge centers generated upon interaction with the cluster medium, which governs the overall survival time of the cluster along the duration of the laser pulse and simultaneously facilitates the extraction of laser energy by the inner ionized electrons. Thus, under nanosecond laser conditions, the properties of the cluster constituent, i.e. ionization energy and nature of bonding within cluster constituents, govern the initial multiphoton ionization step, which ultimately regulates the efficiency of the laser-cluster interaction process.

Table 3. Laser wavelengths and their corresponding ponderomotive energies at different laser intensities used in the present work

355 nm 532 nm 1064 nm

Laser intensity (W/cm2)

Ponderomotive energy (eV)

1.4 × 109 7.4 × 109 4.5 × 109 1.1 × 1010 1.4 × 1010

1.65 × 105 8.7 × 105 1.19 × 104 2.9 × 104 1.48 × 103

Average no. of collisions in 1 ns 8.45 8.45 5.64 5.64 2.82

× 105 × 105 × 105 × 105 × 105

Energy gained by inner ionized electron (eV)

Remarks

13.9 73.5 67.1 163.5 417.3

Fig. 1 Fig. 6(a) Fig. 2 Fig. 6(b) Fig. 5

The total energy gained by an electron is derived from the product of the ponderomotive energy and the average number of collisions experienced by the caged electron in ~1 ns

51




P. Sharma, S. Das and R. K. Vatsa

CONCLUSIONS The interaction of THF clusters with nanosecond laser pulses has been studied at 355, 532 and 1064 nm over an intensity range of 109 to 1010 W/cm2. Along the laser wavelength range studied, the THF clusters exhibited distinct photochemical behaviour. At 355 nm, the clusters were found to undergo multiphoton dissociation/ionization, while, at 532 nm, they gave rise to multiply charged atomic ions upon Coulomb explosion of excessively ionized cluster. Finally, at 1064 nm, the clusters were found to exhibit a Coulomb explosion phenomenon at a laser intensity of ~1010 W/cm2 leading to the generation of hydrogen-like [C]5+ atomic ions, while no evidence for ionization was observed at 109 W/cm2. The phenomenon shows strong wavelength dependence and the extent of the observed charge state was found to increase with increasing wavelength, signifying the efficiency of laser-cluster coupling at longer wavelengths. Based on these studies, it can be concluded that secondary ionization processes following inner ionization are highly efficient in the overall ionization of clusters. Our results suggest that the mechanism which leads to cluster ionization and Coulomb explosion is driven by complex multistep processes, which are strongly coupled, i.e. multiphoton ionization of the cluster followed by energization of electrons (via an IBS process), resulting in multiple ionization of the cluster, which subsequently undergoes Coulomb explosion. An optimum number of charge centers was found to be essential for efficient laser-cluster interaction. Thus, the nature of the cluster constituents, which govern the threshold laser intensity for the initiation of multiphoton ionization and the number of initial charge centers generated within the cluster, plays a crucial role.

Acknowledgements The authors thank Dr V. K. Jain, Head, Chemistry Division, and Dr B. N. Jagatap, Director, Chemistry Group, for constant encouragement. Shri. P. M. Badani is acknowledged for his help during some of the initial experiments.

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Nanosecond electric pulses deprive zinc ions of carboxypeptidase G2.

Towards acousto-optic tissue imaging with nanosecond laser pulses.

Laser-induced breakdown spectroscopy (LIBS) analysis of laser processing in active crystal with nanosecond laser pulses.

Efficient generation of THz pulses with 0.4 mJ energy.

Impact of external medium conductivity on cell membrane electropermeabilization by microsecond and nanosecond electric pulses.

Photodissociation at various wavelengths: fragmentation studies of oxazine 170 using nanosecond laser pulses.

Nonlinear optical properties of near-infrared region Ag2S quantum dots pumped by nanosecond laser pulses.

Efficient laser generation of Lamb waves.

Efficient intracellular delivery of molecules with high cell viability using nanosecond-pulsed laser-activated carbon nanoparticles.

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Controlled oxide films formation by nanosecond laser pulses for color marking.

Efficient single-cell poration by microsecond laser pulses.

Cell electrofusion using nanosecond electric pulses.

Efficient broadband 400 nm noncollinear second-harmonic generation of chirped femtosecond laser pulses in BBO and LBO.

Fabrication of porous metal nanoparticles and microbumps by means of nanosecond laser pulses focused through the fiber microaxicon.

Generation of ultra-large-bandwidth X-ray free-electron-laser pulses with a transverse-gradient undulator.

Infrared nanosecond pulsed laser irradiation of stainless steel: micro iron-oxide zones generation.

Magnetisation switching of FePt nanoparticle recording medium by femtosecond laser pulses.

Fiber grating compression of giant-chirped nanosecond pulses from an ultra-long nanotube mode-locked fiber laser.

Time-resolved absolute measurements by electro-optic effect of giant electromagnetic pulses due to laser-plasma interaction in nanosecond regime.

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Micro- and nanoparticle generation during nanosecond laser ablation: correlation between mass and optical emissions.

Assembling hierarchical cluster solids with atomic precision.

Aligned copper nanorod arrays for highly efficient generation of intense ultra-broadband THz pulses.