CHEMPHYSCHEM COMMUNICATIONS DOI: 10.1002/cphc.201300700
Excited-State Dynamics of the 2-Methylallyl Radical Jçrg Herterich, Thiemo Gerbich, and Ingo Fischer*[a] The excited-state dynamics of hydrocarbon radicals is not well understood and only a few species have been studied.[1] In many radicals, the electronically excited states are short-lived due to rapid internal conversion (IC). This fast deactivation can be used to prepare internally hot radicals for further studies of their unimolecular reactions.[2] Reactions are then described within the framework of statistical theories of chemical reactivity,[3] an approach that requires the dissociation step itself to be rate-limiting. One of the best-studied radicals is allyl, C3H5.[4] Its excited states were characterized by multiphoton ionization (MPI)[5] while the dynamics was explored by time-resolved phototoelectron spectroscopy and H-atom photofragment Doppler spectroscopy. The large amount of information derived from experiment and theory turned allyl into a model for radical dynamics. The experiments showed that the excited electronic states deactivate within roughly 20 ps or less.[6] Subsequent dissociation of hot ground state allyl leads, predominately, to the formation of allene.[7] In addition, the minor channel leading to methyl + acetylene has been observed.[8] Recent experiments focused on the influence of a methyl substituent on the excited states. MPI-, high-resolution photoelectron- and Hatom photofragment spectra of the C4H7 isomers 1- and 2methylallyl (1- and 2-MA) were reported by Bach, Chen and coworkers.[9] They confirmed and extended earlier experimental studies on 2-MA by Callear and Lee,[10] Hudgens and Dulcey,[11] and C.-C. Chen et al.[12] Here we investigate the excited-state dynamics of the B-state of 2-methylallyl (2-MA) by time-resolved photoionization with a ps-laser. Research on resonantly stabilized small radicals such as allyl or methylallyl is not only conducted because of a fundamental interest in reaction dynamics, but also because such radicals can accumulate in a reactive environment and are observed in combustion.[13] Studies on the isolated radicals yield information on their reactions that is important in kinetic models of combustion processes. For example, biodiesel often contains molecules with C=C double bonds (e.g. fatty acid esters).[14] Abstraction of Hatoms then leads to alkylated allyl radicals, because the CH bonds at the allylic sites are particularly weak. The 2-MA radical is generated by pyrolysis from the corresponding bromide, according to Scheme 1. Assuming a freely rotating methyl group, the radical can be described in the C2v point group and the B-state is of 2A1 symmetry (2A’ in Cs). It can also be described as a 3 s Rydberg-state. The transition from the 2A2 electronic ground state of 2-MA (2A“ in Cs) is thus symmetry forbidden in a one-photon process, although some [a] J. Herterich, T. Gerbich, Prof. Dr. I. Fischer Universitt Wrzburg Institute of Physical and Theoretical Chemistry Am Hubland, 97074 Wrzburg (Germany) E-mail: [email protected]
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Scheme 1. 2-methylallyl (2-MA) is generated by pyrolysis from 3-bromo-2methyl-1-propene.
bands become accessible by vibronic coupling. In particular the B-state origin cannot be excited by one photon; therefore, we employed both one- and two-photon excitation to study a range of vibronic bands. Since individual vibronic bands were resolved and assigned in the MPI spectrum obtained with ns-lasers,[9b] a ps-laser system is particularly suitable to investigate the dynamics of these bands. In Figure 1 a [2 + 1’] the MPI spectrum of 2-MA is
Figure 1. [2+1’] MPI spectrum of 2-MA using a tunable picosecond laser.
depicted, obtained by tuning the output of a ps-OPG (optical parametric generator over the excitation region. For ionization, one photon of the third harmonic of a ps-Nd:YLF laser at 351 nm was sufficient, because the ionization energy was measured to be 7.877 eV.[9b] As visible a number of bands can be resolved and in particular the B 00 origin band is clearly discernible. On the other hand the transitions into the 181 (methyl in-plane bend) and 171 (CCC bend) bands cannot be clearly separated, due to a combination of laser bandwidth, rotational temperature and possibly also intensity effects. A third band at roughly + 880 cm1 has not been unambiguously assigned in the ns-spectrum. An overtone of n17, the 171181 combination band or the n16 fundamental might contribute to this band. Ns-work using pyrolysis for generation of 2-MA reported a full width at half maximum (FWHM) of 35 cm1 for some bands,[9a] while experiments that relied on radical generation ChemPhysChem 2013, 14, 3906 – 3908
3906
CHEMPHYSCHEM COMMUNICATIONS by photolysis yielded radicals with lower internal temperatures.[9b] In the one-photon REMPI spectrum (not shown) several further bands can be identified by comparison with the nsspectrum. For all the peaks observed in the one- and two-photon MPI spectra, the lifetimes were obtained from time-delay scans. Some selected scans are depicted in Figures 2 (two-photon excitation) and 3 (one-photon excitation). The solid line repre-
www.chemphyschem.org Table 1. Measured lifetimes for various vibronic bands of the B 2A1 state of 2-methylallyl. All lifetimes are accurate to within 1 ps. Band
lexc [nm][a]
MPI [cm1][13]
lifetime [ps]
00 261 251 171 241 172 251181 251171 251172
521.9 260.0 257.9 515.7 256.6 509.9 255.2 254.8 252.1
38342 38526 38790 38800 38965 39205 39177 39254 39691
14.0 7.6 12.5 12.5 10.4 8.0 9.1 10.3 8.4
alternative assignments[b]
181 171181/161
251161
[a] Excitation wavelength used in the experiment. [b] Due to the laser bandwidths, several modes might contribute to a band.
Figure 2. Selected time-delay traces for vibrational levels of the B-state of 2MA, recorded in a two-photon excitation.
252.1 nm (one-photon excitation), several modes might contribute due to the 25 cm1 bandwidth of the excitation pulse and the rotational temperature. Together with the structured MPI spectrum, the observed time constants are in agreement with an internal conversion (IC) to the electronic ground state. Most likely IC proceeds in a two-step process via the lower-lying A-state, as concluded for C3H5 based on experiments on the deuterated radical.[6a] In allyl the A-state exhibits shorter lifetimes than the B-state and the second step in this deactivation cannot be time-resolved with a ps-laser. No ionization from the final state of the deactivation is observed in Figure 1 and Figure 2, presumably due to small Franck–Condon factors for ionization from highly excited vibrational states. H-atom loss on the ns time scale was observed by Gasser et al. in 2-MA upon excitation of several of the bands monitored above and interpreted to be due to a statistical dissociation from the electronic ground state.[9a] The lifetimes presented here are in full agreement with this conclusion. Note that there are no low-lying quartet states in most hydrocarbon radicals, so intersystem crossing is not a relevant deactivation pathway Compared with allyl, the lifetimes observed for 2-MA are shorter by roughly 30–50 %. For example, for the B-state origin of allyl, a 20 ps decay was observed,[6b] while 14 ps are measured for 2-MA. IC rates are often described by the Golden Rule[15] according to Equation (1). kIC V 2 FC 1
Figure 3. Selected time-delay traces for vibrational levels of the B-state of 2MA recorded in a one-photon excitation.
sents a fit which was obtained by convoluting the 6 ps instrument function with a monoexponential decay. The pump– probe signal decreased to zero at long delay times, that is, no offset was observed. The lifetimes determined for the various bands are summarized in Table 1. To some of the bands investigated, for example at 509.9 nm (two-photon excitation) and 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ð1Þ
where V represents the electronic coupling, FC the Franck– Condon factors between the initial excited state and the accepting mode of the lower electronic state, and 1 is the rovibrational density of states of the final electronic state, that is, the A-state. Since the character of the electronic states is the same in allyl and 2-MA, we assume that V is similar in the two molecules. On the other hand, 1 will be increased considerably upon adding a methyl group. This should be easily verified, but calculations of 1 turned out to be difficult for two reasons: First the excitation energy of the A-state of 2-MA is only known from computations and second the torsional mode of ChemPhysChem 2013, 14, 3906 – 3908
3907
CHEMPHYSCHEM COMMUNICATIONS 2-MA has to be treated as a free internal rotor at higher excess energies. As a consequence even slight changes in the input parameters have a large influence on 1, rendering quantitative conclusions questionable. Nevertheless the decrease in lifetime upon methylation is in agreement with a simple Golden Rule picture. With increasing excitation energy a slight decrease of lifetimes is apparent, which is again in agreement with a dominant influence of the density of states on the deactivation. We want to point out that the effect of methyl substitution on the photochemical deactivation is not straightforward. The nonradiative decay in alkenes, for example, has been found to become slower when an H atom is replaced by a methyl group.[16] This shows that the FC term in [Eq. (1)] can be influenced appreciably by the frequency reduction in an accepting mode by methyl substitution. Note also that the CCC bond angle increases in 2-MA upon methyl substitution from 1218 in allyl to 1258, which will also have some influence on the FC term. To summarize, in 2-methylallyl this effect seems to be less relevant than in other molecules and the nonradiative decay is accelerated compared to allyl by the higher density of states.
www.chemphyschem.org output itself was utilized as the probe pulse. For the [1 + 1’] experiments we used collimated but unfocused laser beams with a pulse power of about 60–80 mJ (pump) and 200–250 mJ (probe), corresponding to intensities on the order of 1011 W·m2. For the [2 + 1’] spectra the laser output from the OPG (400–500 mJ) and the third harmonic (500 mJ) was focused slightly away from the ionization region by a 300 mm focal length lens. The bandwidth of the pump pulse is around 25 cm1 at 500 nm, while the probe pulse is assumed to be transform-limited. The probe pulse was sent over a variable delay line mounted on a computer-controlled stepper motor, the output of the pump pulse was sent over a fixed delay line. Subsequently the pump- and probe-beam were recombined in a dichroic mirror and directed into the ionization region. Ion signals were recorded in a microchannelplate detector, digitized in a digital storage oscilloscope and then transferred to a personal computer. Most delay traces represents an average over 8 scans, in each scan 75 laser shots were recorded per data point. Only for the 00 band 16 scans were averaged.
Acknowledgements This work was financially supported by the Deutsche Forschungsgemeinschaft under contract FI 575/9–1.
Conclusions The photophysics in 2-methylallyl after UV excitation has been studied by time-resolved photoionization. The lifetimes of several vibronic bands in the B-state of 2-MA have been determined and the results have been compared to previous experiments on allyl, C3H5. The bands decay by internal conversion to the electronic ground state, converting electronic to internal excitation. Previous work by Gasser et al. found a statistical Hatom loss on the ground-state potential energy surface. The time-resolved experiments strongly support the present interpretation of the photochemistry and photodissociation dynamics of 2-MA. The lifetimes are shorter by roughly 30–50 % as compared to allyl. Within a Golden Rule picture, this difference can be explained by the increase in the density of states due to the additional methyl group.
Experimental Section The experiments were carried out in a standard molecular beam apparatus equipped with a Wiley-McLaren time-of-flight mass spectrometer described previously.[17] Briefly, we generated a clean beam of 2-methylallyl radicals by pyrolysis of 3-bromo-2-methylpropene, obtained from Sigma Aldrich. The precursor is seeded in 1.2 to 1.4 bar of argon and expanded through a SiC tube of 1 mm diameter mounted onto a solenoid valve with a nozzle of 0.8 mm diameter, operating at 10 Hz. The tube is electrically heated to around 800–1000 8C to split the thermochemically weakest bond. For the time-resolved experiments we employed a novel picosecond laser system. The third harmonic output (7–8 mJ at 351 nm) of an Nd:YLF laser (EKSPLA, PG401-SH-YLF) is directed into an optical parametric generator (OPG, EXPLA PG-401-SH), which provides tunable light for excitation of 2-methylallyl. A pulse duration of 4 ps was measured for the OPG output at 800 nm. In several experiments the visible output of the OPG was frequency doubled for one-photon excitation of the radicals. A part of the 3rd harmonic
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Keywords: excited-state dynamics · internal conversion · radicals · reactive intermediates · time-resolved spectroscopy [1] a) C. Alcaraz, D. Schrçder, I. Fischer, Radical Chemistry in the Gas Phase, in Encyclopedia of Radicals in Chemistry, Biology and Materials, Vol. 1, (Eds.: C. Chatgilialoglu, A. Studer), John Wiley & Sons, Chichester, 2012, pp. 477 – 502; b) M. Zierhut, B. Noller, T. Schultz, I. Fischer, J. Chem. Phys. 2005, 122, 094302; c) B. Noller, R. Maksimenka, I. Fischer, M. Armone, B. Engels, L. Poisson, J.-M. Mestdagh, J. Phys. Chem. A 2007, 111, 1771 – 1779. [2] I. Fischer, Chimia 2000, 54, 96 – 102. [3] T. Baer, W. L. Hase, Unimolecular Reaction Dynamics, Oxford University Press, New York, 1996. [4] I. Fischer, P. Chen, J. Phys. Chem. A 2002, 106, 4291 – 4300. [5] M. Gasser, J. A. Frey, J. M. Hostettler, A. Bach, P. Chen, J. Phys. Chem. A 2010, 114, 4704 – 4711. [6] a) T. Schultz, J. S. Clarke, H.-J. Deyerl, T. Gilbert, I. Fischer, Faraday Discuss. 2000, 115, 17 – 31; b) T. Schultz, I. Fischer, J. Chem. Phys. 1997, 107, 8197. [7] H.-J. Deyerl, I. Fischer, P. Chen, J. Chem. Phys. 1999, 110, 1450 – 1462. [8] D. Stranges, M. Stemmler, X. Yang, J. D. Chesko, A. G. Suits, Y. T. Lee, J. Chem. Phys. 1998, 109, 5372 – 5382. [9] a) M. Gasser, A. Bach, P. Chen, Phys. Chem. Chem. Phys. 2008, 10, 1133 – 1138; b) M. Gasser, J. A. Frey, J. M. Hostettler, A. Bach, J. Mol. Spectrosc. 2010, 263, 93 – 100; c) M. Gasser, J. A. Frey, J. M. Hostettler, A. Bach, Chem. Commun. 2011, 47, 301 – 303. [10] A. B. Callear, H. K. Lee, Trans. Faraday Soc. 1968, 64, 2017 – 2022. [11] J. W. Hudgens, C. S. Dulcey, J. Phys. Chem. 1985, 89, 1505. [12] C.-C. Chen, H.-C. Wu, C.-M. Tseng, Y.-H. Yang, Y.-T. Chen, J. Chem. Phys. 2003, 119, 241 – 250. [13] V. D. Knyazev, I. R. Slagle, J. Phys. Chem. A 1998, 102, 8932 – 8940. [14] C. K. Westbrook, Annu. Rev. Phys. Chem. 2013, 64, 201 – 219. [15] M. Bixon, J. Jortner, J. Chem. Phys. 1968, 48, 715 – 726. [16] J.-M. Mestdagh, J.-P. Visticot, M. Elhanine, B. Soep, J. Chem. Phys. 2000, 113, 237 – 248. [17] C. Schon, W. Roth, I. Fischer, J. Pfister, C. Kaiser, R. F. Fink, B. Engels, Phys. Chem. Chem. Phys. 2010, 12, 9339 – 9346. Received: July 30, 2013 Published online on November 4, 2013
ChemPhysChem 2013, 14, 3906 – 3908
3908
Excited-State Dynamics of the 2-Methylallyl Radical Jçrg Herterich, Thiemo Gerbich, and Ingo Fischer*[a] The excited-state dynamics of hydrocarbon radicals is not well understood and only a few species have been studied.[1] In many radicals, the electronically excited states are short-lived due to rapid internal conversion (IC). This fast deactivation can be used to prepare internally hot radicals for further studies of their unimolecular reactions.[2] Reactions are then described within the framework of statistical theories of chemical reactivity,[3] an approach that requires the dissociation step itself to be rate-limiting. One of the best-studied radicals is allyl, C3H5.[4] Its excited states were characterized by multiphoton ionization (MPI)[5] while the dynamics was explored by time-resolved phototoelectron spectroscopy and H-atom photofragment Doppler spectroscopy. The large amount of information derived from experiment and theory turned allyl into a model for radical dynamics. The experiments showed that the excited electronic states deactivate within roughly 20 ps or less.[6] Subsequent dissociation of hot ground state allyl leads, predominately, to the formation of allene.[7] In addition, the minor channel leading to methyl + acetylene has been observed.[8] Recent experiments focused on the influence of a methyl substituent on the excited states. MPI-, high-resolution photoelectron- and Hatom photofragment spectra of the C4H7 isomers 1- and 2methylallyl (1- and 2-MA) were reported by Bach, Chen and coworkers.[9] They confirmed and extended earlier experimental studies on 2-MA by Callear and Lee,[10] Hudgens and Dulcey,[11] and C.-C. Chen et al.[12] Here we investigate the excited-state dynamics of the B-state of 2-methylallyl (2-MA) by time-resolved photoionization with a ps-laser. Research on resonantly stabilized small radicals such as allyl or methylallyl is not only conducted because of a fundamental interest in reaction dynamics, but also because such radicals can accumulate in a reactive environment and are observed in combustion.[13] Studies on the isolated radicals yield information on their reactions that is important in kinetic models of combustion processes. For example, biodiesel often contains molecules with C=C double bonds (e.g. fatty acid esters).[14] Abstraction of Hatoms then leads to alkylated allyl radicals, because the CH bonds at the allylic sites are particularly weak. The 2-MA radical is generated by pyrolysis from the corresponding bromide, according to Scheme 1. Assuming a freely rotating methyl group, the radical can be described in the C2v point group and the B-state is of 2A1 symmetry (2A’ in Cs). It can also be described as a 3 s Rydberg-state. The transition from the 2A2 electronic ground state of 2-MA (2A“ in Cs) is thus symmetry forbidden in a one-photon process, although some [a] J. Herterich, T. Gerbich, Prof. Dr. I. Fischer Universitt Wrzburg Institute of Physical and Theoretical Chemistry Am Hubland, 97074 Wrzburg (Germany) E-mail: [email protected]
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Scheme 1. 2-methylallyl (2-MA) is generated by pyrolysis from 3-bromo-2methyl-1-propene.
bands become accessible by vibronic coupling. In particular the B-state origin cannot be excited by one photon; therefore, we employed both one- and two-photon excitation to study a range of vibronic bands. Since individual vibronic bands were resolved and assigned in the MPI spectrum obtained with ns-lasers,[9b] a ps-laser system is particularly suitable to investigate the dynamics of these bands. In Figure 1 a [2 + 1’] the MPI spectrum of 2-MA is
Figure 1. [2+1’] MPI spectrum of 2-MA using a tunable picosecond laser.
depicted, obtained by tuning the output of a ps-OPG (optical parametric generator over the excitation region. For ionization, one photon of the third harmonic of a ps-Nd:YLF laser at 351 nm was sufficient, because the ionization energy was measured to be 7.877 eV.[9b] As visible a number of bands can be resolved and in particular the B 00 origin band is clearly discernible. On the other hand the transitions into the 181 (methyl in-plane bend) and 171 (CCC bend) bands cannot be clearly separated, due to a combination of laser bandwidth, rotational temperature and possibly also intensity effects. A third band at roughly + 880 cm1 has not been unambiguously assigned in the ns-spectrum. An overtone of n17, the 171181 combination band or the n16 fundamental might contribute to this band. Ns-work using pyrolysis for generation of 2-MA reported a full width at half maximum (FWHM) of 35 cm1 for some bands,[9a] while experiments that relied on radical generation ChemPhysChem 2013, 14, 3906 – 3908
3906
CHEMPHYSCHEM COMMUNICATIONS by photolysis yielded radicals with lower internal temperatures.[9b] In the one-photon REMPI spectrum (not shown) several further bands can be identified by comparison with the nsspectrum. For all the peaks observed in the one- and two-photon MPI spectra, the lifetimes were obtained from time-delay scans. Some selected scans are depicted in Figures 2 (two-photon excitation) and 3 (one-photon excitation). The solid line repre-
www.chemphyschem.org Table 1. Measured lifetimes for various vibronic bands of the B 2A1 state of 2-methylallyl. All lifetimes are accurate to within 1 ps. Band
lexc [nm][a]
MPI [cm1][13]
lifetime [ps]
00 261 251 171 241 172 251181 251171 251172
521.9 260.0 257.9 515.7 256.6 509.9 255.2 254.8 252.1
38342 38526 38790 38800 38965 39205 39177 39254 39691
14.0 7.6 12.5 12.5 10.4 8.0 9.1 10.3 8.4
alternative assignments[b]
181 171181/161
251161
[a] Excitation wavelength used in the experiment. [b] Due to the laser bandwidths, several modes might contribute to a band.
Figure 2. Selected time-delay traces for vibrational levels of the B-state of 2MA, recorded in a two-photon excitation.
252.1 nm (one-photon excitation), several modes might contribute due to the 25 cm1 bandwidth of the excitation pulse and the rotational temperature. Together with the structured MPI spectrum, the observed time constants are in agreement with an internal conversion (IC) to the electronic ground state. Most likely IC proceeds in a two-step process via the lower-lying A-state, as concluded for C3H5 based on experiments on the deuterated radical.[6a] In allyl the A-state exhibits shorter lifetimes than the B-state and the second step in this deactivation cannot be time-resolved with a ps-laser. No ionization from the final state of the deactivation is observed in Figure 1 and Figure 2, presumably due to small Franck–Condon factors for ionization from highly excited vibrational states. H-atom loss on the ns time scale was observed by Gasser et al. in 2-MA upon excitation of several of the bands monitored above and interpreted to be due to a statistical dissociation from the electronic ground state.[9a] The lifetimes presented here are in full agreement with this conclusion. Note that there are no low-lying quartet states in most hydrocarbon radicals, so intersystem crossing is not a relevant deactivation pathway Compared with allyl, the lifetimes observed for 2-MA are shorter by roughly 30–50 %. For example, for the B-state origin of allyl, a 20 ps decay was observed,[6b] while 14 ps are measured for 2-MA. IC rates are often described by the Golden Rule[15] according to Equation (1). kIC V 2 FC 1
Figure 3. Selected time-delay traces for vibrational levels of the B-state of 2MA recorded in a one-photon excitation.
sents a fit which was obtained by convoluting the 6 ps instrument function with a monoexponential decay. The pump– probe signal decreased to zero at long delay times, that is, no offset was observed. The lifetimes determined for the various bands are summarized in Table 1. To some of the bands investigated, for example at 509.9 nm (two-photon excitation) and 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ð1Þ
where V represents the electronic coupling, FC the Franck– Condon factors between the initial excited state and the accepting mode of the lower electronic state, and 1 is the rovibrational density of states of the final electronic state, that is, the A-state. Since the character of the electronic states is the same in allyl and 2-MA, we assume that V is similar in the two molecules. On the other hand, 1 will be increased considerably upon adding a methyl group. This should be easily verified, but calculations of 1 turned out to be difficult for two reasons: First the excitation energy of the A-state of 2-MA is only known from computations and second the torsional mode of ChemPhysChem 2013, 14, 3906 – 3908
3907
CHEMPHYSCHEM COMMUNICATIONS 2-MA has to be treated as a free internal rotor at higher excess energies. As a consequence even slight changes in the input parameters have a large influence on 1, rendering quantitative conclusions questionable. Nevertheless the decrease in lifetime upon methylation is in agreement with a simple Golden Rule picture. With increasing excitation energy a slight decrease of lifetimes is apparent, which is again in agreement with a dominant influence of the density of states on the deactivation. We want to point out that the effect of methyl substitution on the photochemical deactivation is not straightforward. The nonradiative decay in alkenes, for example, has been found to become slower when an H atom is replaced by a methyl group.[16] This shows that the FC term in [Eq. (1)] can be influenced appreciably by the frequency reduction in an accepting mode by methyl substitution. Note also that the CCC bond angle increases in 2-MA upon methyl substitution from 1218 in allyl to 1258, which will also have some influence on the FC term. To summarize, in 2-methylallyl this effect seems to be less relevant than in other molecules and the nonradiative decay is accelerated compared to allyl by the higher density of states.
www.chemphyschem.org output itself was utilized as the probe pulse. For the [1 + 1’] experiments we used collimated but unfocused laser beams with a pulse power of about 60–80 mJ (pump) and 200–250 mJ (probe), corresponding to intensities on the order of 1011 W·m2. For the [2 + 1’] spectra the laser output from the OPG (400–500 mJ) and the third harmonic (500 mJ) was focused slightly away from the ionization region by a 300 mm focal length lens. The bandwidth of the pump pulse is around 25 cm1 at 500 nm, while the probe pulse is assumed to be transform-limited. The probe pulse was sent over a variable delay line mounted on a computer-controlled stepper motor, the output of the pump pulse was sent over a fixed delay line. Subsequently the pump- and probe-beam were recombined in a dichroic mirror and directed into the ionization region. Ion signals were recorded in a microchannelplate detector, digitized in a digital storage oscilloscope and then transferred to a personal computer. Most delay traces represents an average over 8 scans, in each scan 75 laser shots were recorded per data point. Only for the 00 band 16 scans were averaged.
Acknowledgements This work was financially supported by the Deutsche Forschungsgemeinschaft under contract FI 575/9–1.
Conclusions The photophysics in 2-methylallyl after UV excitation has been studied by time-resolved photoionization. The lifetimes of several vibronic bands in the B-state of 2-MA have been determined and the results have been compared to previous experiments on allyl, C3H5. The bands decay by internal conversion to the electronic ground state, converting electronic to internal excitation. Previous work by Gasser et al. found a statistical Hatom loss on the ground-state potential energy surface. The time-resolved experiments strongly support the present interpretation of the photochemistry and photodissociation dynamics of 2-MA. The lifetimes are shorter by roughly 30–50 % as compared to allyl. Within a Golden Rule picture, this difference can be explained by the increase in the density of states due to the additional methyl group.
Experimental Section The experiments were carried out in a standard molecular beam apparatus equipped with a Wiley-McLaren time-of-flight mass spectrometer described previously.[17] Briefly, we generated a clean beam of 2-methylallyl radicals by pyrolysis of 3-bromo-2-methylpropene, obtained from Sigma Aldrich. The precursor is seeded in 1.2 to 1.4 bar of argon and expanded through a SiC tube of 1 mm diameter mounted onto a solenoid valve with a nozzle of 0.8 mm diameter, operating at 10 Hz. The tube is electrically heated to around 800–1000 8C to split the thermochemically weakest bond. For the time-resolved experiments we employed a novel picosecond laser system. The third harmonic output (7–8 mJ at 351 nm) of an Nd:YLF laser (EKSPLA, PG401-SH-YLF) is directed into an optical parametric generator (OPG, EXPLA PG-401-SH), which provides tunable light for excitation of 2-methylallyl. A pulse duration of 4 ps was measured for the OPG output at 800 nm. In several experiments the visible output of the OPG was frequency doubled for one-photon excitation of the radicals. A part of the 3rd harmonic
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Keywords: excited-state dynamics · internal conversion · radicals · reactive intermediates · time-resolved spectroscopy [1] a) C. Alcaraz, D. Schrçder, I. Fischer, Radical Chemistry in the Gas Phase, in Encyclopedia of Radicals in Chemistry, Biology and Materials, Vol. 1, (Eds.: C. Chatgilialoglu, A. Studer), John Wiley & Sons, Chichester, 2012, pp. 477 – 502; b) M. Zierhut, B. Noller, T. Schultz, I. Fischer, J. Chem. Phys. 2005, 122, 094302; c) B. Noller, R. Maksimenka, I. Fischer, M. Armone, B. Engels, L. Poisson, J.-M. Mestdagh, J. Phys. Chem. A 2007, 111, 1771 – 1779. [2] I. Fischer, Chimia 2000, 54, 96 – 102. [3] T. Baer, W. L. Hase, Unimolecular Reaction Dynamics, Oxford University Press, New York, 1996. [4] I. Fischer, P. Chen, J. Phys. Chem. A 2002, 106, 4291 – 4300. [5] M. Gasser, J. A. Frey, J. M. Hostettler, A. Bach, P. Chen, J. Phys. Chem. A 2010, 114, 4704 – 4711. [6] a) T. Schultz, J. S. Clarke, H.-J. Deyerl, T. Gilbert, I. Fischer, Faraday Discuss. 2000, 115, 17 – 31; b) T. Schultz, I. Fischer, J. Chem. Phys. 1997, 107, 8197. [7] H.-J. Deyerl, I. Fischer, P. Chen, J. Chem. Phys. 1999, 110, 1450 – 1462. [8] D. Stranges, M. Stemmler, X. Yang, J. D. Chesko, A. G. Suits, Y. T. Lee, J. Chem. Phys. 1998, 109, 5372 – 5382. [9] a) M. Gasser, A. Bach, P. Chen, Phys. Chem. Chem. Phys. 2008, 10, 1133 – 1138; b) M. Gasser, J. A. Frey, J. M. Hostettler, A. Bach, J. Mol. Spectrosc. 2010, 263, 93 – 100; c) M. Gasser, J. A. Frey, J. M. Hostettler, A. Bach, Chem. Commun. 2011, 47, 301 – 303. [10] A. B. Callear, H. K. Lee, Trans. Faraday Soc. 1968, 64, 2017 – 2022. [11] J. W. Hudgens, C. S. Dulcey, J. Phys. Chem. 1985, 89, 1505. [12] C.-C. Chen, H.-C. Wu, C.-M. Tseng, Y.-H. Yang, Y.-T. Chen, J. Chem. Phys. 2003, 119, 241 – 250. [13] V. D. Knyazev, I. R. Slagle, J. Phys. Chem. A 1998, 102, 8932 – 8940. [14] C. K. Westbrook, Annu. Rev. Phys. Chem. 2013, 64, 201 – 219. [15] M. Bixon, J. Jortner, J. Chem. Phys. 1968, 48, 715 – 726. [16] J.-M. Mestdagh, J.-P. Visticot, M. Elhanine, B. Soep, J. Chem. Phys. 2000, 113, 237 – 248. [17] C. Schon, W. Roth, I. Fischer, J. Pfister, C. Kaiser, R. F. Fink, B. Engels, Phys. Chem. Chem. Phys. 2010, 12, 9339 – 9346. Received: July 30, 2013 Published online on November 4, 2013
ChemPhysChem 2013, 14, 3906 – 3908
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