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Structured emission induced by ArF laser excitation of ketene in a molecular beam
19 August 1994
ELSEVIER
CHEMICAL PHYSICS LETTERS
Chemical Physics Letters 226 ( 1994) 300-304
Structured emission induced by ArF laser excitation of ketene in a molecular beam Javier Ruiz, Margarita Martin Institutede QuimicaFisica, CSIC, Serrano I1 9, 28006 Madrid, Spain Received 11 May 1994; in final form 2 June 1994
Abstract Structured emission at wavelengths 540-690 nm is observed following ArF laser excitation of a molecular beam of ketene. The emission depends linearly on laser energy. The fluorescence shows a fast risetime and biexponential decay. Under the lowest collision regime studied the fast and slow components of the decay have time constants of 1.9 f 0.7 ps and 7.2 f 0.8 ps respectively. The observed emission is tentatively assigned to fluorescence from vibrationally excited CHr ( ‘Br ); formation of the latter in the photodissociation of ketene has been predicted theoretically but no previous experimental observation has been reported.
1. Introduction
Recent experimental and theoretical work has provided a great deal of information on the photodissociation of ketene at energies ranging from the moleo ular dissociation threshold [ I] up to the regions lying 5600 cm-’ above the singlet dissociation threshold [ 2,3 1. Direct studies have obtained branching ratios for the production of the lowest-lying triplet and singlet states of the methylene photofragment [ 41; product state distributions at different ketene excitation energies have also been measured [ 3 1. The region of excitation energies corresponding to the strong absorption band of ketene between 2 15 and 190 nm has also been explored, although to much less extent. The main experimental and theoretical results have been summarized by Ball et al. [ 5 ] and Liu et al. [ 6 1. At excitation energies corresponding to absorption of one 193 nm laser photon the most relevant results available are briefly outlined here: at that energy the high vibrational levels of the ‘B, electronic state of ketene are accessible; two dissociation channels have
been identified: dissociation into CHCO ( 2A”) + H(‘S) [ 71 and dissociation into CH2 and CO (X ‘Z+ ). Regarding the latter channel, experimental [ 8 ] and theoretical [ 9 ] work has concluded that dissociation of CH&O ( ‘Br ) involves a bent geometry for the CC0 bond with CH2 out of the molecular plane; along that dissociation pathway, state correlation diagrams show that CH2C0 ( ‘B I ) correlates with the products CH2( ‘Br ) +CO( *Z+). However, experiments in which several Torr of ketene are photolyzed at 193 nm have failed to observe any dispersed fluorescence in the spectral region where emission from CH2( ‘B1) would be expected [ 8 1. In an attempt to observe the state population distribution in which methylene photofragments are formed in the ArF laser dissociation of ketene we have carried out the photolysis in a molecular beam. The results shown in this work indicate that, under the low collision regime typical of a molecular beam, a prompt fluorescence is observed that could be attributed to formation of CH2 ( ‘B, ).
0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZOOO9-2614(94)00723-4
J. Ruin, M. Martin /Chemical Physics Letters 226 (1994) 300-304
2. Experimental
Samples of ketene or ketene diluted in He as the carrier gas were expanded to a vacuum chamber through a pulsed solenoid valve (General Valve Corporation, 0.05 cm of diameter, 1 ms pulse duration). Background pressure in the vacuum chamber was 1 X 10s6 Torr. The output of an ArF excimer laser ( 15 ns pulse duration; 193 nm ) , propagating perpendicular to the direction of the molecular beam, was focused to a spot of x 0.1 X 0.4 cm*, about 1 cm below the nozzle; typical laser powers per pulse ranged from 30 to 90 mJ/cm*. LIF experiments could also be performed; a XeCl-pumped dye laser (rhodamine 6G dye in methanol) giving 0.5 mJ per pulse and bandwidth 0.2 cm-’ co-propagated with the photolysis laser beam. Spontaneous (or laser-induced) fluorescence originated in the interaction region, was collected orthogonal to the propagation of laser and molecular beams by a quartz lens, spectrally filtered by appropriate cut-off or interference filters and viewed by a photomultiplier (Hammamatsu R928, spectral response 185-930 nm). Signals were sent to gated integrators and boxcar averagers (Stanford Research Systems SRS245) and digitized by an a/d converter built into a programmable unit which allows control and proper timing of trigger pulses [ 10 1. Time-resolved fluorescence was recorded by a 40 MHz digital oscilloscope Tektronix 2430A (risetime of 12 ns and vertical resolution of 8 bits). Typically 256 traces were averaged before being transferred to a microcomputer to be stored and analyzed. Care was taken in all the experiments to filter out fluorescence from electronically excited CH photofragments which are known to be formed in the multiphoton dissociation of ketene at 193 nm [ 111. Ketene was prepared by pyrolysis of acetic anhydride and purified twice by trap-to-trap distillation from 163 to 77 K. Mass-spectrometric analysis of samples indicated impurity levels of less than 3W. Ketene stagnation pressures for the different experimental conditions were achieved keeping the samples at different temperatures; slush baths at 179, 189 and 200 K were employed. For experiments with He as the carrier gas, 1.5 atm of He was bubbled through a bulb containing ketene at the temperature of the selected slush bath.
301
3. Results Fluorescence was observed following ArF laser excitation of ketene in a molecular beam. Under low spectral resolution the emission shows some structure in the region between 540 and 690 nm peaking near 650 nm. The dependence of the time-integrated fluorescence signal on the laser energy was studied. Emission, filtered by a glass plate was collected by a low resolution, high intensity Bausch and Lomb monochromator centred near the maximum of the emission at 650 f 45 nm. Laser energies ranged from about 1 to 10 mJ. A logarithmic plot of the emission dependence on laser energy is represented in Fig. 1; the straight line best fitting the plot has slope equal to 1.07, indicating that a single-photon laser excitation process is likely to be responsible for the observed fluorescence. The fluorescence temporal decay in several spectral regions was studied. Most of the measurements were carried out in the spectral region near 640 nm selecting the fluorescence by an interference filter (Specac 640FS 10-50, bandwidth 10 nm). Decays were also recorded collecting broadband fluorescence around 650 nm with a bandwidth of + 45 nm. Time resolved traces were recorded at different backing pressures of ketene. O_
-1;
slope=
1.07
-b In
laser
energy
Fig. 1. Logarithmic plot of time integrated fluorescence signal at 650 f 45 nm, obtained after ArF laser excitation of ketene, versus laser energy.
J. Ruiz, M. Martin /Chemical Physics Letters 226 (1994) 300-304
302
The recorded traces rise in a time equal or faster than the risetime of the oscilloscope and decay in several microseconds. Semilogarithmic plots indicate that the observed decays cannot be fitted to single exponent&. Good fittings to two exponentials are obtained in all cases except when broadband emission (bandwidth It 45 nm) is collected. Fig. 2 shows time-resolved emission isolated at 640 f 5 nm; in these experiments a crude estimation of the ketene pressure at the interaction region with the laser beam can be made by assuming ideal gas conditions for the expansion; for stagnation pressures of 40 Torr of pure ketene and a nozzle diameter of 0.05 cm, the terminal velocity and gas beam intensity can be roughly evaluated leading to an estimation of the pressure at the interaction region of a few mTorr [ 121. The best fitting of the decay to two exponentials and residuals are also shown in Fig. 2. Decay rates obtained from the analysis of traces recorded under similar conditions showed reasonable agreement; traces can be well fitted to two exponen8,
1 cl
tials, one faster decay with a time constant of 1.9 f 0.7 ps and a slower decay with a time constant of 7.2 f 0.8 ps. The main errors are carried by the faster component to the decay due to the distortion of the initial part of the traces, caused by electromagnetic and other sources of noise. Time-resolved fluorescence isolated at 640 f 5 nm was also recorded at higher ketene backing pressures. Pressures of ketene before the nozzle of x 160 Torr shorten the time constants of both decay components by a factor of sz0.8. When 1.5 atm of He is bubbled through liquid samples of ketene kept at 179 K, faster decays are observed: 1.2 f0.7 and 4.5 f0.5 ps are measured for the fast and slow decay components, respectively. Finally we mention that unsuccessful attempts were made to observe LIF from any CH2 ( ‘Ai ) that might be formed in the ArF laser dissociation of ketene. The detection limits of our experimental system were checked by recording the well-known LIF spectrum of CHI( ‘Ai ) produced by photolyzing ketene at 308 nm i; the dye laser was scanned from 588.7 to 590.8 nm and the LIF signal collected at 640 + 5 nm. At 193 nm, under a similar backing pressure of ketene, similar spatial and temporal overlap of excimer and dye laser beams, estimating similar densities of photolyzed molecules in the beam and similar detection conditions, no LIF could be observed from CH2 ( ‘Ai ) superimposed to the background prompt fluorescence.
4. Discussion t /
psec
24
I b
Fig. 2. (a) Time-resolved fluorescence isolated at 640 ? 5 nm obtained following ArF laser excitation of ketene; (b) semilogarithmic plot and best fitting to two exponential decays; (c) residuals.
The species giving rise to the observed fluorescence will be considered in the present discussion. The possibility of attributing the observed emission to impurities in the ketene samples can be safely disregarded; reproducible results were obtained with samples obtained from a different synthesis. Moreover, the analysis of samples by mass spectrometry indicated that the main impurity is acetic anhydride which has zero or low vapour pressure at the temperatures of the slush baths used in the experiments. Fluorescence from undissociated ketene is highly ’ Assignment of the equivalent absorption spectrum can be found inRef. [13].
J. Ruiz, M. Martin /Chemical PhysicsLetters226 (I 994) 300-304
unlikely; the available data indicate that dissociation rates of ketene increase rapidly with energy above the first singlet dissociation threshold; time constants of picoseconds have been measured at moderately high excess energy [ 2,4] and are expected to be shorter at the higher energies attained under excitation at 193 nm. Regarding the ketene dissociation channel leading to formation of CHCO( ‘A”) which has been positively identified in the photolysis at 193 nm, measurements of the ketene bond dissociation energy give DH$,,(H-CHCO) = 37039 f 734 cm-’ [ 141; therefore, the maximum excess energy available to CHCO is x 14700 cm-’ which would be too small to account for the observed emission at wavelengths shorter than 680 nm. Finally, the possibility of assigning the observed emission to methylene in the ‘B, state remains to be considered. Upon ArF laser excitation the excess energy over the first singlet dissociation threshold is z 2 16 17 cm-‘; therefore, population of several vibrational levels of CH2 ( ‘B, ) would be accessible and, on thermochemical grounds, excitation of stretching and bending modes, up to ~4 or x20 for v1 and v2 respectively, would be energetically possible [ 15- 18 1. The low spectral resolution used to record the observed emission precludes us from assigning the spectral features although their positions are compatible with transitions belonging to the CH2 (‘A,-‘B, ) system; more difficulties concerning the analysis of the emission arise from the fact that Franck-Condon factors and Einstein coefficients are only available for transitions between vibrational levels (vi = 0, v2= 1318, v3=O) in CH2( ‘B, ) and the lowest vibrational levels of the ‘Ai state, that can be excited in LIF or absorption measurements [ 17,191. The temporal decays of the observed fluorescence are also compatible with lifetimes and collisional deactivation rates of CH2 ( ‘B L) reported in the literature. Several transitions of the vibronic band ‘Bi (0, 14,0)-‘Ai (0, 1,O) with K, = 0 and rotational levels J= 2-8 have energies in the spectral region 640 k 5 nm [ 131; for the latter, experimental lifetimes of single rotational lines have been reported to be in the range 4.9 to 7.5 us [ 181; falling in the same spectral region are transitions from levels with K,=3 and J= 3-6 belonging to the vibronic band ‘Bi (0, 13,0)*Ai(0, 0, 0) for which theoretical calculations pre-
303
diet lifetimes of 15 us [ 18 1; the times reported in this work for the slow component of the fluorescence would also be in qualitative agreement with experimental determinations obtained for rotationally unresolved emission starting in the vibrational level (0, 14,0) of methylene ‘B, [20]. The fast decay component to the fluorescence signal observed in the present experiments could be related to rotational population transfer which has been found to be fast in highly excited vibrational states of methylene ‘B1 [ 2 1 ] ; however the unresolved nature of the emission and the complex collisional processes which operate in the removal of excited methylene [ 2 1 ] prevent any further analysis of the present results.
5. Conclusions and final remarks On the basis of the temporal and spectral characteristics and the observed dependence on laser energy of the emission reported in this work, we tentatively conclude that it can be assigned to fluorescence from vibrationally excited CH2( 'B, ) formed in the photodissociation of ketene at 193 nm. Work to improve fluorescence detection sensitivity of our experimental system, aimed to record the dispersed fluorescence under higher resolution and to extend the spectrum to shorter and longer wavelengths is currently underway. Experiments with deuterated ketene could provide ways to assign the observed emission. Work is also in progress to find appropriate time delays between pump and probe lasers in order to attenuate prompt background fluorescence which could mask laser induced fluorescence from CH2( ‘Ai) photofragments.
Acknowledgement Financial support from DGICYT (PB90-007 1) is acknowledged. JR thanks the CAM (Comunidad Autonoma de Madrid) for a scholarship. The authors also thank Dr. I. Garcia-Moreno for useful comments to this work, Mr. M. Rodriguez for technical help and Ms. L. Abad and Dr. V. Herrero for making available a computer program to calculate molecular beam properties.
304
J. Ruiz, M. Martin /Chemical Physics Letters 226 (1994) 300-304
References [ 1 ] I-C. Chen and C.B. Moore, J. Phys. Chem. 94 (1990) 263. [ 21 E.D. Potter, M. Gruebele, L.R. Khundkar and A.H. Zewail, Chem. Phys. Letters 164 (1989) 463. [3] I. Garcfa-Moreno, E.R. Lovejoy and C.B. Moore, J. Chem. Phys., in press, and references therein. [ 41 SK. Kim, Y.S. Choi, C.D. Pibel, Q. Zheng and C.B. Moore, J. Chem. Phys. 94 ( 1991) 1954. [5] SM. Ball, G. Hancock and M.R. Heal, J. Chem. Sot. Faraday Trans. 90 (1994) 523. [6] X. Liu, S.G. Westre, J.D. Getty and P.B. Kelly, Chem. Phys. Letters 188 (1992) 42. [ 71 KG. Unfried, G.P. Glass and R.F. Curl, Chem. Phys. Letters 177 (1991) 33. [8] B.I. Sonobe and R.N. Rosenfeld, J. Am. Chem. Sot. 105 (1983) 7528. [ 91 W.D. Allen and H.F. Schaefer III, J. Chem. Phys. 89 ( 1988) 329. [lo] M. Rodriguez, M. Martin and J. Ruiz, Meas. Sci. Technol. 3 (1992) 229.
] 111 J. Luque, J. Ruiz and M. Martin, Chem. Phys. Letters 202 (1993) 179. [ 121 D.R. Miller, in: Atomic and molecular beam methods, Vol. 1. Free jet sources, ed. G. Stoles (Oxford Univ. Press, Oxford, 1988) pp. 17-33. [ 131 H. Petek, D.J. Nesbitt, D.C. Darwin and C.B. Moore, J. Chem. Phys. 86 ( 1987) 1172. [ 141 J.M. Oakes, M.E. Jones, V.M. Bierbaum and G.B. Ellison, J. Phys. Chem. 87 (1983) 4810. [ 151 G. Hetzberg and J.W.C. Johns, Proc. Roy. Sot. A 295 (1966) 107. [ 161 G. Duxbury and Ch. Jungen, Mol. Phys. 63 ( 1988) 981. [ 171 W.H. Green Jr., N.C. Handy, P.J. Knowles and S. Carter, J. Chem. Phys. 94 (1991) 118. [ 181 I. Garcia-Moreno, E.R. Lovejoy, C.B. Moore and G. Duxbury, J. Chem. Phys. 98 (1993) 873. [ 191 I. Garcia-Moreno and C.B. Moore, J. Chem. Phys. 99 (1993) 6429. [ 201 M.N.R. Ashfold, M.A. Fullstone, G. Hancock and G.W. Ketley, Chem. Phys. 55 (1981) 245. [21] G.V. Hartland, D. Quiu and H.-L. Dai, J. Chem. Phys. 98 ( 1993) 6906.
ELSEVIER
CHEMICAL PHYSICS LETTERS
Chemical Physics Letters 226 ( 1994) 300-304
Structured emission induced by ArF laser excitation of ketene in a molecular beam Javier Ruiz, Margarita Martin Institutede QuimicaFisica, CSIC, Serrano I1 9, 28006 Madrid, Spain Received 11 May 1994; in final form 2 June 1994
Abstract Structured emission at wavelengths 540-690 nm is observed following ArF laser excitation of a molecular beam of ketene. The emission depends linearly on laser energy. The fluorescence shows a fast risetime and biexponential decay. Under the lowest collision regime studied the fast and slow components of the decay have time constants of 1.9 f 0.7 ps and 7.2 f 0.8 ps respectively. The observed emission is tentatively assigned to fluorescence from vibrationally excited CHr ( ‘Br ); formation of the latter in the photodissociation of ketene has been predicted theoretically but no previous experimental observation has been reported.
1. Introduction
Recent experimental and theoretical work has provided a great deal of information on the photodissociation of ketene at energies ranging from the moleo ular dissociation threshold [ I] up to the regions lying 5600 cm-’ above the singlet dissociation threshold [ 2,3 1. Direct studies have obtained branching ratios for the production of the lowest-lying triplet and singlet states of the methylene photofragment [ 41; product state distributions at different ketene excitation energies have also been measured [ 3 1. The region of excitation energies corresponding to the strong absorption band of ketene between 2 15 and 190 nm has also been explored, although to much less extent. The main experimental and theoretical results have been summarized by Ball et al. [ 5 ] and Liu et al. [ 6 1. At excitation energies corresponding to absorption of one 193 nm laser photon the most relevant results available are briefly outlined here: at that energy the high vibrational levels of the ‘B, electronic state of ketene are accessible; two dissociation channels have
been identified: dissociation into CHCO ( 2A”) + H(‘S) [ 71 and dissociation into CH2 and CO (X ‘Z+ ). Regarding the latter channel, experimental [ 8 ] and theoretical [ 9 ] work has concluded that dissociation of CH&O ( ‘Br ) involves a bent geometry for the CC0 bond with CH2 out of the molecular plane; along that dissociation pathway, state correlation diagrams show that CH2C0 ( ‘B I ) correlates with the products CH2( ‘Br ) +CO( *Z+). However, experiments in which several Torr of ketene are photolyzed at 193 nm have failed to observe any dispersed fluorescence in the spectral region where emission from CH2( ‘B1) would be expected [ 8 1. In an attempt to observe the state population distribution in which methylene photofragments are formed in the ArF laser dissociation of ketene we have carried out the photolysis in a molecular beam. The results shown in this work indicate that, under the low collision regime typical of a molecular beam, a prompt fluorescence is observed that could be attributed to formation of CH2 ( ‘B, ).
0009-2614/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDZOOO9-2614(94)00723-4
J. Ruin, M. Martin /Chemical Physics Letters 226 (1994) 300-304
2. Experimental
Samples of ketene or ketene diluted in He as the carrier gas were expanded to a vacuum chamber through a pulsed solenoid valve (General Valve Corporation, 0.05 cm of diameter, 1 ms pulse duration). Background pressure in the vacuum chamber was 1 X 10s6 Torr. The output of an ArF excimer laser ( 15 ns pulse duration; 193 nm ) , propagating perpendicular to the direction of the molecular beam, was focused to a spot of x 0.1 X 0.4 cm*, about 1 cm below the nozzle; typical laser powers per pulse ranged from 30 to 90 mJ/cm*. LIF experiments could also be performed; a XeCl-pumped dye laser (rhodamine 6G dye in methanol) giving 0.5 mJ per pulse and bandwidth 0.2 cm-’ co-propagated with the photolysis laser beam. Spontaneous (or laser-induced) fluorescence originated in the interaction region, was collected orthogonal to the propagation of laser and molecular beams by a quartz lens, spectrally filtered by appropriate cut-off or interference filters and viewed by a photomultiplier (Hammamatsu R928, spectral response 185-930 nm). Signals were sent to gated integrators and boxcar averagers (Stanford Research Systems SRS245) and digitized by an a/d converter built into a programmable unit which allows control and proper timing of trigger pulses [ 10 1. Time-resolved fluorescence was recorded by a 40 MHz digital oscilloscope Tektronix 2430A (risetime of 12 ns and vertical resolution of 8 bits). Typically 256 traces were averaged before being transferred to a microcomputer to be stored and analyzed. Care was taken in all the experiments to filter out fluorescence from electronically excited CH photofragments which are known to be formed in the multiphoton dissociation of ketene at 193 nm [ 111. Ketene was prepared by pyrolysis of acetic anhydride and purified twice by trap-to-trap distillation from 163 to 77 K. Mass-spectrometric analysis of samples indicated impurity levels of less than 3W. Ketene stagnation pressures for the different experimental conditions were achieved keeping the samples at different temperatures; slush baths at 179, 189 and 200 K were employed. For experiments with He as the carrier gas, 1.5 atm of He was bubbled through a bulb containing ketene at the temperature of the selected slush bath.
301
3. Results Fluorescence was observed following ArF laser excitation of ketene in a molecular beam. Under low spectral resolution the emission shows some structure in the region between 540 and 690 nm peaking near 650 nm. The dependence of the time-integrated fluorescence signal on the laser energy was studied. Emission, filtered by a glass plate was collected by a low resolution, high intensity Bausch and Lomb monochromator centred near the maximum of the emission at 650 f 45 nm. Laser energies ranged from about 1 to 10 mJ. A logarithmic plot of the emission dependence on laser energy is represented in Fig. 1; the straight line best fitting the plot has slope equal to 1.07, indicating that a single-photon laser excitation process is likely to be responsible for the observed fluorescence. The fluorescence temporal decay in several spectral regions was studied. Most of the measurements were carried out in the spectral region near 640 nm selecting the fluorescence by an interference filter (Specac 640FS 10-50, bandwidth 10 nm). Decays were also recorded collecting broadband fluorescence around 650 nm with a bandwidth of + 45 nm. Time resolved traces were recorded at different backing pressures of ketene. O_
-1;
slope=
1.07
-b In
laser
energy
Fig. 1. Logarithmic plot of time integrated fluorescence signal at 650 f 45 nm, obtained after ArF laser excitation of ketene, versus laser energy.
J. Ruiz, M. Martin /Chemical Physics Letters 226 (1994) 300-304
302
The recorded traces rise in a time equal or faster than the risetime of the oscilloscope and decay in several microseconds. Semilogarithmic plots indicate that the observed decays cannot be fitted to single exponent&. Good fittings to two exponentials are obtained in all cases except when broadband emission (bandwidth It 45 nm) is collected. Fig. 2 shows time-resolved emission isolated at 640 f 5 nm; in these experiments a crude estimation of the ketene pressure at the interaction region with the laser beam can be made by assuming ideal gas conditions for the expansion; for stagnation pressures of 40 Torr of pure ketene and a nozzle diameter of 0.05 cm, the terminal velocity and gas beam intensity can be roughly evaluated leading to an estimation of the pressure at the interaction region of a few mTorr [ 121. The best fitting of the decay to two exponentials and residuals are also shown in Fig. 2. Decay rates obtained from the analysis of traces recorded under similar conditions showed reasonable agreement; traces can be well fitted to two exponen8,
1 cl
tials, one faster decay with a time constant of 1.9 f 0.7 ps and a slower decay with a time constant of 7.2 f 0.8 ps. The main errors are carried by the faster component to the decay due to the distortion of the initial part of the traces, caused by electromagnetic and other sources of noise. Time-resolved fluorescence isolated at 640 f 5 nm was also recorded at higher ketene backing pressures. Pressures of ketene before the nozzle of x 160 Torr shorten the time constants of both decay components by a factor of sz0.8. When 1.5 atm of He is bubbled through liquid samples of ketene kept at 179 K, faster decays are observed: 1.2 f0.7 and 4.5 f0.5 ps are measured for the fast and slow decay components, respectively. Finally we mention that unsuccessful attempts were made to observe LIF from any CH2 ( ‘Ai ) that might be formed in the ArF laser dissociation of ketene. The detection limits of our experimental system were checked by recording the well-known LIF spectrum of CHI( ‘Ai ) produced by photolyzing ketene at 308 nm i; the dye laser was scanned from 588.7 to 590.8 nm and the LIF signal collected at 640 + 5 nm. At 193 nm, under a similar backing pressure of ketene, similar spatial and temporal overlap of excimer and dye laser beams, estimating similar densities of photolyzed molecules in the beam and similar detection conditions, no LIF could be observed from CH2 ( ‘Ai ) superimposed to the background prompt fluorescence.
4. Discussion t /
psec
24
I b
Fig. 2. (a) Time-resolved fluorescence isolated at 640 ? 5 nm obtained following ArF laser excitation of ketene; (b) semilogarithmic plot and best fitting to two exponential decays; (c) residuals.
The species giving rise to the observed fluorescence will be considered in the present discussion. The possibility of attributing the observed emission to impurities in the ketene samples can be safely disregarded; reproducible results were obtained with samples obtained from a different synthesis. Moreover, the analysis of samples by mass spectrometry indicated that the main impurity is acetic anhydride which has zero or low vapour pressure at the temperatures of the slush baths used in the experiments. Fluorescence from undissociated ketene is highly ’ Assignment of the equivalent absorption spectrum can be found inRef. [13].
J. Ruiz, M. Martin /Chemical PhysicsLetters226 (I 994) 300-304
unlikely; the available data indicate that dissociation rates of ketene increase rapidly with energy above the first singlet dissociation threshold; time constants of picoseconds have been measured at moderately high excess energy [ 2,4] and are expected to be shorter at the higher energies attained under excitation at 193 nm. Regarding the ketene dissociation channel leading to formation of CHCO( ‘A”) which has been positively identified in the photolysis at 193 nm, measurements of the ketene bond dissociation energy give DH$,,(H-CHCO) = 37039 f 734 cm-’ [ 141; therefore, the maximum excess energy available to CHCO is x 14700 cm-’ which would be too small to account for the observed emission at wavelengths shorter than 680 nm. Finally, the possibility of assigning the observed emission to methylene in the ‘B, state remains to be considered. Upon ArF laser excitation the excess energy over the first singlet dissociation threshold is z 2 16 17 cm-‘; therefore, population of several vibrational levels of CH2 ( ‘B, ) would be accessible and, on thermochemical grounds, excitation of stretching and bending modes, up to ~4 or x20 for v1 and v2 respectively, would be energetically possible [ 15- 18 1. The low spectral resolution used to record the observed emission precludes us from assigning the spectral features although their positions are compatible with transitions belonging to the CH2 (‘A,-‘B, ) system; more difficulties concerning the analysis of the emission arise from the fact that Franck-Condon factors and Einstein coefficients are only available for transitions between vibrational levels (vi = 0, v2= 1318, v3=O) in CH2( ‘B, ) and the lowest vibrational levels of the ‘Ai state, that can be excited in LIF or absorption measurements [ 17,191. The temporal decays of the observed fluorescence are also compatible with lifetimes and collisional deactivation rates of CH2 ( ‘B L) reported in the literature. Several transitions of the vibronic band ‘Bi (0, 14,0)-‘Ai (0, 1,O) with K, = 0 and rotational levels J= 2-8 have energies in the spectral region 640 k 5 nm [ 131; for the latter, experimental lifetimes of single rotational lines have been reported to be in the range 4.9 to 7.5 us [ 181; falling in the same spectral region are transitions from levels with K,=3 and J= 3-6 belonging to the vibronic band ‘Bi (0, 13,0)*Ai(0, 0, 0) for which theoretical calculations pre-
303
diet lifetimes of 15 us [ 18 1; the times reported in this work for the slow component of the fluorescence would also be in qualitative agreement with experimental determinations obtained for rotationally unresolved emission starting in the vibrational level (0, 14,0) of methylene ‘B, [20]. The fast decay component to the fluorescence signal observed in the present experiments could be related to rotational population transfer which has been found to be fast in highly excited vibrational states of methylene ‘B1 [ 2 1 ] ; however the unresolved nature of the emission and the complex collisional processes which operate in the removal of excited methylene [ 2 1 ] prevent any further analysis of the present results.
5. Conclusions and final remarks On the basis of the temporal and spectral characteristics and the observed dependence on laser energy of the emission reported in this work, we tentatively conclude that it can be assigned to fluorescence from vibrationally excited CH2( 'B, ) formed in the photodissociation of ketene at 193 nm. Work to improve fluorescence detection sensitivity of our experimental system, aimed to record the dispersed fluorescence under higher resolution and to extend the spectrum to shorter and longer wavelengths is currently underway. Experiments with deuterated ketene could provide ways to assign the observed emission. Work is also in progress to find appropriate time delays between pump and probe lasers in order to attenuate prompt background fluorescence which could mask laser induced fluorescence from CH2( ‘Ai) photofragments.
Acknowledgement Financial support from DGICYT (PB90-007 1) is acknowledged. JR thanks the CAM (Comunidad Autonoma de Madrid) for a scholarship. The authors also thank Dr. I. Garcia-Moreno for useful comments to this work, Mr. M. Rodriguez for technical help and Ms. L. Abad and Dr. V. Herrero for making available a computer program to calculate molecular beam properties.
304
J. Ruiz, M. Martin /Chemical Physics Letters 226 (1994) 300-304
References [ 1 ] I-C. Chen and C.B. Moore, J. Phys. Chem. 94 (1990) 263. [ 21 E.D. Potter, M. Gruebele, L.R. Khundkar and A.H. Zewail, Chem. Phys. Letters 164 (1989) 463. [3] I. Garcfa-Moreno, E.R. Lovejoy and C.B. Moore, J. Chem. Phys., in press, and references therein. [ 41 SK. Kim, Y.S. Choi, C.D. Pibel, Q. Zheng and C.B. Moore, J. Chem. Phys. 94 ( 1991) 1954. [5] SM. Ball, G. Hancock and M.R. Heal, J. Chem. Sot. Faraday Trans. 90 (1994) 523. [6] X. Liu, S.G. Westre, J.D. Getty and P.B. Kelly, Chem. Phys. Letters 188 (1992) 42. [ 71 KG. Unfried, G.P. Glass and R.F. Curl, Chem. Phys. Letters 177 (1991) 33. [8] B.I. Sonobe and R.N. Rosenfeld, J. Am. Chem. Sot. 105 (1983) 7528. [ 91 W.D. Allen and H.F. Schaefer III, J. Chem. Phys. 89 ( 1988) 329. [lo] M. Rodriguez, M. Martin and J. Ruiz, Meas. Sci. Technol. 3 (1992) 229.
] 111 J. Luque, J. Ruiz and M. Martin, Chem. Phys. Letters 202 (1993) 179. [ 121 D.R. Miller, in: Atomic and molecular beam methods, Vol. 1. Free jet sources, ed. G. Stoles (Oxford Univ. Press, Oxford, 1988) pp. 17-33. [ 131 H. Petek, D.J. Nesbitt, D.C. Darwin and C.B. Moore, J. Chem. Phys. 86 ( 1987) 1172. [ 141 J.M. Oakes, M.E. Jones, V.M. Bierbaum and G.B. Ellison, J. Phys. Chem. 87 (1983) 4810. [ 151 G. Hetzberg and J.W.C. Johns, Proc. Roy. Sot. A 295 (1966) 107. [ 161 G. Duxbury and Ch. Jungen, Mol. Phys. 63 ( 1988) 981. [ 171 W.H. Green Jr., N.C. Handy, P.J. Knowles and S. Carter, J. Chem. Phys. 94 (1991) 118. [ 181 I. Garcia-Moreno, E.R. Lovejoy, C.B. Moore and G. Duxbury, J. Chem. Phys. 98 (1993) 873. [ 191 I. Garcia-Moreno and C.B. Moore, J. Chem. Phys. 99 (1993) 6429. [ 201 M.N.R. Ashfold, M.A. Fullstone, G. Hancock and G.W. Ketley, Chem. Phys. 55 (1981) 245. [21] G.V. Hartland, D. Quiu and H.-L. Dai, J. Chem. Phys. 98 ( 1993) 6906.