Crossed-Molecular-Beam Observations of Vibrational Energy Transfer

Chapter 16

Crossed-Molecular-Beam Observations of Vibrational Energy Transfer in Only Moderately Excited Glyoxal Molecules Downloaded by UNIV OF ARIZONA on January 11, 2013 | http://pubs.acs.org Publication Date: June 10, 1997 | doi: 10.1021/bk-1997-0678.ch016

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Samuel M . Clegg, Brian D. Gilbert , Shao-ping Lu , and Charles S. Parmenter Department of Chemistry, Indiana University, Bloomington, IN 47405 Crossed molecular beams were used to study rotational and rovibrational inelastic scattering of S glyoxal (CHO-CHO)fromD , N , CO and C H . A laser is used to prepare glyoxal to the 0°, K' = 0 state. The rotational and rovibrational states excited by inelastic scattering were monitored by dispersedfluorescence.The collision partners were chosen to investigate the relative influence of the reduced mass versus the interaction potential of the collision pair. The glyoxal + D and glyoxal + He inelastic spectra were nearly identical while the glyoxal + D and glyoxal + H spectra were clearly different. This indicates that the inelastic scattering is dominated by the kinematics rather than by the details of the interaction potentials of the various collision partners. Glyoxal collisions with N , CO and C H , each with a mass of 28 amu, were also nearly identical, further evidence that inelastic scattering is controlled by the reduced mass of the collision pair. The study of the relative rotational and rovibrational channel competition has also been extended to include the glyoxal + N , CO and C H relative cross sections. These new data continue to demonstrate that the rotational versus rovibrational channel competition strongly depends on the reduced mass of the collision pair. 1

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Crossed molecular beams with a laser pump-dispersed fluorescence probe has been shown to be an effective method of monitoring state-to-state vibrational energy transfer (VET). This method was initially used to study inelastic scattering from the BOJ state of iodine (1-4). An already extensive study from the \ state of irans-glyoxal is still ongoing (5-8). Inelastic scattering from glyoxal has become a benchmark for fully quantal three-dimensional scattering calculations (9-12). In this report, we describe the results of experiments designed to investigate the relative influence of the reduced mass of the collision pair versus the interaction potential in controlling the rotational and rovibrational energy transfer. This was l

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Current address: Department of Chemistry and Physics, Coastal Carolina University, P.O. Box 261954, Conway, SC 29528-6054

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Current address: Innovative Lasers Corporation, 3280 East Hemisphere Loop #120, Tucson, AZ 85706 © 1997 American Chemical Society In Highly Excited Molecules; Mullin, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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HIGHLY EXCITED MOLECULES

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examined by adding rotationally resolved inelastic scattering spectra from glyoxal collisions with deuterium (D , 4 amu), nitrogen (N , 28 amu), carbon monoxide (CO, 28 amu) and ethylene ( C ^ , 28 amu) to results already published. Crossed molecular beams offer many advantages for V E T study that are not possible in room temperature beam experiments. A primary asset is the relatively precise definition of collision energies. Additionally, the cold rotational temperatures from the beam expansion allows laser preparation of a specific initial rovibrational state, as well as monitoring with Si - S dispersed fluorescence of all important inelastic scattering channels with rotational resolution. Details of the V E T channel selectivity and competition in glyoxal may be seen with the vibrational energy level diagram of Figure 1 and in its accompanying 0° and 7 rotational energy level diagram The diagram shows all 27 Sj vibrational levels through 1200 cm" , many of which may be pumped selectively. Initial state preparation in previous studies selected zero or little angular momentum (KR = 0) about the "a" axis in each of the vibrational energy levels shown in bold, namely 0°, 7 , 5 and 8 (5-8). A l l of the principal inelastic scattering channels were observed by collecting fluorescence from the rotational and rovibrational Κ states populated by inelastic scattering. Regardless of the initial vibrational level, V E T involving Δ υ - ±1 was the only channel observed even though many other energy levels are energetically accessible. The rotational energy level diagram shows that the 0° and 7 rotational manifolds overlap at ΔΕ > 233 cm" resulting in competition between rotational and rovibrational inelastic scattering. That competition was initially studied from glyoxal pumped to the level 0°, K ' = 0, a state we shall call 0°K°, in collision with five target gases, namely H (2 amu), D (4 amu), He (4 amu), Kr (84 amu) and cyclohexane (84 amu) (5-8). Gilbert et al. reported that the competition among rotational and rovibrational channels depended primarily on the mass of the target gas (6). Their conclusion was based on the observation of fluorescence spectra where rotational resolution is not achieved. The inelastic scattering spectra from glyoxal 0°K° + He (4 amu) and D (4 amu) were qualitatively similar, while qualitative differences were observed between the glyoxal 0°K° + H (2 amu) and D (4 amu) lower resolution spectra. These results indicated that the mass of the target gas has dominant control over the V E T channel competition while the interaction potential is much less important (6). A unique aspect of our new work is to do the more difficult experiment of using higher resolution where the individual rotational channels are resolved. We have compared rotationally resolved glyoxal 0°K° + D inelastic scattering spectrum with the published glyoxal 0°K° + H and He spectra. Inelastic scattering spectra from glyoxal 0°K° + N , C O and C H4, all with a mass of 28 amu, are also used for a more definitive probe of the mass control of V E T . A l l three inelastic scattering spectra should be similar if the mass truly exerts the dominant influence over the relative cross sections. These new data will also better characterize the rotational versus rovibrational competition as the target gas mass changes. The rotational and rovibrational relative cross sections for glyoxal 0°K° + N , C O and QH4 are now added to the published H , He and K r relative cross sections (5-8). 2

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Downloaded by UNIV OF ARIZONA on January 11, 2013 | http://pubs.acs.org Publication Date: June 10, 1997 | doi: 10.1021/bk-1997-0678.ch016

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In Highly Excited Molecules; Mullin, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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CLEGG ET AL.

Crossed-Molecular-Beam Observations

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600 - K = 14

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. 10

ε 1

• 7 K° 200

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FIG. 1. A vibrational energy level diagram (left) of S glyoxal with a rotational diagram (right) showing the Κ = J rotational energy levels that may be observed in inelastic scattering from the 0°, K ' = 0 state of S glyoxal. The bold levels are those so far pumped in published glyoxal experiments. The arrows show the principal vibrationally inelastic scattering channels from each initial levels. Glyoxal is almost a symmetric top with the top "a" axis passing near the oxygen atoms (insert). 1

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Results and Discussion Detailed descriptions of the experimental setup and data analysis used in obtaining the data presented here have been published (2,8). The relative cross sections are extracted by recreating the experimental spectrum with a simulation program. A description of the simulation program has been published (8). Figure 2 contains both simulated (line) and experimental spectra (circles) from glyoxal + D . Each of the peaks to the blue of the dip in the spectra indicates emission from a rotational or rovibrational state that has been excited by the inelastic collision.


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Mass Has Dominant Control of the Relative Inelastic Scattering Cross Sections. The reduced mass of the collision pair and the interaction potential between the collision pair are important parameters in control of inelastic scattering. Inelastic scattering from glyoxal collisions with H , D and He offers a particularly interesting opportunity to investigate the relative influence of the reduced mass versus the interaction potential. The interaction potentials between glyoxal + H and glyoxal + D are identical whereas the masses of the target gases are different by a factor of two. In contrast, the glyoxal + D and glyoxal + He interaction potentials are different and the target gas masses are the same. Rotationally resolved inelastic scattering spectra from glyoxal + H2 and glyoxal + He along with their relative cross sections have been published (8). A new rotationally resolved glyoxal + D inelastic scattering spectrum, shown in Figure 2, can be compared to the glyoxal + H and glyoxal + He rotationally resolved inelastic scattering spectra. Figure 3 contains the superimposed glyoxal + H and glyoxal + D spectra (top) and the glyoxal + D and glyoxal + He spectra (bottom). It is obvious that the H2 and D spectra begin to diverge for emission from 0°K > 9. Émission from 0°K =15-17 can be observed in the glyoxal + D spectrum where rotational structure from glyoxal + H cannot be resolved past K . In contrast, the inelastic scattering spectra for D and He, shown at the bottom of Figure 3, are nearly identical This mass effect was tested further in Κ state resolved experiments with three 28 amu collision partners, namely N , C O and C U . The interaction potential between glyoxal and each of the target gases is different. The inelastic scattering spectra are compared in Figure 4. Close inspection of the three spectra shows how well the rotational structure overlaps. The three spectra are again nearly identical, thus the extracted relative cross sections generally match to within experimental error. Further indication of the insensitivity to the interaction potential comes from theoretical studies of H and He interactions with glyoxal (9-72). The calculated inelastic cross sections for both H and He agree with the experimental relative cross sections even though several interaction potential approximations were made (8,10,13). For example, the same glyoxal + He interaction potential was used in both calculations (9-72). Furthermore, the interaction potential for S glyoxal + He was really a ground electronic state potential constructed from atom-atom pair potentials derived from the ab-initio formaldehyde + He system (9-72). 2

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Competition Between Rotational and Rovibrational Inelastic Scattering. As described elsewhere, experimental relative cross sections can be obtained from the


16. CLEGG ET AL.


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Fig. 2. The OS and 7} bands of S - S fluorescence from glyoxal molecules that have been scattered from the S (0°, K ' = 0) state by collisions with D . Circles show the experimental spectrum while the line is a computer simulation. The maxima are subbands from the K ' states reached by rotationally inelastic scattering and are so labeled. RovibrationaJly inelastic scattering produces emission from K ' states within the 7 level that emits in the 7} band. This emission is also shown at 5x and displaced upward for clarity. l

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FIG. 3. A comparison of the experimentalfluorescencespectra from S glyoxal (0°, K ' = 0) that is inelastically scattered by collision with H , D or He. l

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CLEGG ET AL.

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FIG. 4. A comparison of the experimentalfluorescencespectra from Sj glyoxal (0°, K ' = 0) that is inelastically scattered by collisions with N , CO or C H . 2

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computer simulation of the scattering spectra. Those for glyoxal + D from the present study are shown in Figure 5, where they are compared with H and He cross sections obtained earlier (5,7,8). As can be seen in the display of cross sections against the energy ΔΕ transferred (T - R,V), the relative cross sections for D and He are almost indistinguishable, whereas both sets are clearly different from the cross sections. Figure 6 contains a plot of the relative cross sections for each mass set, (2 amu), He and D (4 amu), N , CO and Q H ^ (28 amu) and Kr and C H (84 amu). The relative cross sections for gases with the same mass are essentially identical, falling within their respective error bars. For convenience, the mass sets listed above will be referred to as H , He, N and Kr where the other gases in each set are implied. 2

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Rotational and Rovibrational Relative Cross Sections versus Target Gas Mass. Figure 7 contains a plot where the relative rotational cross sections are compared. The cross sections decrease more or less exponentially with increasing rotational excitation through the transition 0° K ' = 10 (we shall call it o(0°K )). Beyond that, the relative cross sections o(0°K ) through o(0°K ) have a strong dependence on the target gas mass. The fall off in the glyoxal + H and glyoxal + He relative rotational cross sections from K through K indicates that rotational excitation may be limited by angular momentum (5-8). The maximum initial orbital angular momentum can be estimated classically as L = where μ is the reduced mass of the collision pair, ν is the relative velocity of the collision pair and b is the impact parameter (5,73). The maximum rotational excitation about the "a" axis from collisions with H , He, N and Kr is approximately 14H, 22H, 74R and 122R, respectively. The smaller H and He relative cross sections may, therefore, be the result of angular momentum restrictions. Angular momentum limitations with N and Kr are much too large to be observed in our spectra. A l l of the mass sets of relative rovibrational cross sections show a strong dependence on the target gas mass. The glyoxal + Hj rovibrational cross sections follow the same exponential scaling against ΔΕ as the rotational cross sections. As the target gas mass increases, the 7 rovibrational excitation becomes less competitive from 7 Κ° through 7 K , as shown in Figure 8. Angular momentum again limits the rotational excitation beginning near 7 K . The relative value of the cross sections increases with mass for rovibrational excitation from 7 K through 7 K . 10

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Rotational versus Rovibrational Competition. The rotational energy level diagram in Figure 1 indicates that the amount of energy, ΔΕ, required for rovibrational excitation to l K° is approximately the same as for rotational excitation to 0°K . Here, at ΔΕ « 230 cm" , the rotational versus rovibrational competition becomes particularly interesting. This competition can be observed in the separation of the relative rotational and rovibrational cross sections shown in Figure 6. The relative magnitude of the rotational and rovibrational cross sections depends on the mass of the target gas. The relative rotational and rovibrational cross sections for glyoxal + Kr and glyoxal + N are well separated. The relative rotational and rovibrational cross sections for glyoxal + He are also well separated but the separation is not as large as that for K r and N . The rotational and rovibrational cross sections for glyoxal + H , however, are not separated. l

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CLEGG ETAL.

Crossed-Molecular-Beatn Observations

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FIG. 5. The relative cross sections plotted against the energy transfer ΔΕ (Τ - R,V) for inelastic scattering from S glyoxal (0°, K ' = 0) by H (top) and He (bottom, closed symbols) and D (bottom, open symbols). The circles and squares are the rotational and 7 rovibrational relative cross sections, respectively. x

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FIG. 6. The relative cross sections plotted against the energy transfer ΔΕ (T - R,V) for inelastic scattered S glyoxal (0°, K ' = 0) with various target gases. Cross sections from different gases of the same mass are coincident to within the error bars. The open and closed circles are rotational and 7 rovibrational relative cross sections, respectively.

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CLEGG ET AL.

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CLEGG ET AL.


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Rotational versus rovibrational competition can be summarized by taking the ratio of the sum of the rotational, o(rot), and rovibrational, o(vib), relative cross sections. Figure 9 contains a plot of the experimental o(vib)/o(rot) ratio plotted against target gas mass. The plot shows that the o(vib)/o(rot) ratio decreases smoothly as the target mass increases. The value of each of the ratios is on the order of 0.1. The relative nature of the experimental cross sections does not describe how each mass set changes relative to the others, whereas the absolute theoretical cross sections can. The calculations for H^ and He, at our experimental E , indicate that both o(rot) and o(vib) increase but that o(rot) increases by a larger amount. c n L


Conclusion Crossed molecular beams and a laser pump-dispersed fluorescence probe were used to study rotational and rovibrational inelastic scattering in Si glyoxal. Experiments were designed to characterize the influence of the reduced mass versus the interaction potential in setting the relative cross sections. This characterization was accomplished by comparing the Κ state resolved inelastic scattering spectra as well as the relative cross sections from glyoxal collisions with different target gases where the interaction potential or the target gas mass were similar. When the mass of the target gases were similar, namely glyoxal + D and glyoxal + He, the inelastic scattering spectra and relative cross section were qualitatively similar. When the interaction potentials between the collision pair are identical, namely glyoxal + and glyoxal + D , the spectra were clearly different. This indicates that the reduced mass of the collision pair is far more important than the interaction potential in controlling inelastic scattering. Rotationally resolved inelastic scattering spectra from glyoxal collisions with N , C O and C ^ further indicated that the target gas mass has far more control than the interaction potential in setting the relative cross sections. The inelastic scattering spectra and relative cross sections from collisions with N , CO and C ^ were nearly identical. The rotational energy level diagram in Figure 1 shows that the 0° and 7 rotational manifolds overlap. The most interesting part of the rotational and rovibrational competition was observed when there were approximately equal amounts of energy going into rotational and rovibrational excitation. Details of this competition between rotational and rovibrational energy transfer have been extended by the addition of the new N , CO and data to the published 1^, He and Kr relative cross sections. The competition between rotational and rovibrational energy transfer has been shown to depend on the mass of the target gas. As the mass of the target gas increases, the separation between the relative rotational and rovibrational cross sections where equal amounts of energy are exchanged become larger. 2

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Acknowledgment We are grateful for the financial support of this work provided by the National Science Foundation and by the Petroleum Research Foundation administered by the American Chemical Society.


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HIGHLY EXCITED MOLECULES 0.135 0.130

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ι . . . . ι 40 60

80

ι . . I 100

Atomic/Molecular Mass (amu) FIG. 9. A plot of o(vib)/o(rot) against ΔΕ for the inelastic scattering of Sj glyoxal (0°, K ' = 0) by various target gases.

Literature Cited 1. Krajnovich, D.J.; Butz, K.W.; Du, H.; Parmenter, C.S. J. Phys. Chem. 1988, 92, 1388. 2. Krajnovich, D.J.; Butz, K.W.; Du, H.; Parmenter, C.S. J. Chem. Phys. 1989, 91, 7705. 3. Krajnovich, D.J.; Butz, K.W.; Du, H.; Parmenter, C.S. J. Chem. Phys. 1989, 91, 7725. 4. Du, H.; Krajnovich, D.J.; Parmenter, C.S. J. Phys. Chem. 1989, 95, 2104. 5. Butz, K.W.; Du, H.; Krajnovich, D.J.; Parmenter, C.S. J. Chem. Phys. 1988, 89, 4680. 6. Gilbert, B.D.; Parmenter, C.S.; Krajnovich, D.J. J. Chem. Phys. 1994, 101, 7440. 7. Gilbert, B.D.; Parmenter, C.S.; Krajnovich, D.J. J. Phys. Chem. 1994, 98, 7116. 8. Gilbert, B.D.; Parmenter, C.S.; Krajnovich, D.J. J. Chem. Phys. 1994, 101, 7423. 9. Clary, D.C.; Dateo, C.E. Chem. Phys. Lett. 1989, 154, 62. 10. Kroes, G.-J.; Rettschnick, R.P.H.; Clary, D.C. Chem. Phys. 1990, 145, 359. 11. Kroes, G.-J.; Rettschnick, R.P.H.; Dateo, C.E.; Clary, D.C. J. Chem. Phys. 1990, 93, 287. 12. Kroes, G.-J.; Rettschnick, R.P.H. J. Chem. Phys. 1991, 94, 360. 13. Levine, R.D.; Bernstein, R.B. Molecular Reaction Dynamics; Oxford University: New York, NY, 1974, pp 32-33.


Crossed-Molecular-Beam Observations of Vibrational Energy Transfer

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