J. Phys. Chem. 1961, 85, 2711-2713
2711
Laser Production of Jet-Cooled Radicals. Methoxy and Methoxy-Argon D. E. Powers, J. B. Hopklns, and R. E. Smalley* Rlce Quantum Institute and Lkpafiment of Chemistry, Rice University, Houston,
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7700 1 (Received: July 24, 198 1)
A novel pulsed supersonic nozzle has been developed to produce intense cold beams of free radicals formed by laser photolysis in a high-pressure, fast-flow region within the nozzle. Photolysis of methyl benzoate in this apparatus efficiently formed the methoxy radical. Laser-induced fluorescence excitation spectra were obtained for the A 2A1 X ?Esystem of the jet-cooled methoxy radical and the van der Waah complex CH,OAr. Rotational analysis revealed that both of these radicals retain an electronic angular momentum of 0.4 h in the 2Eground electronic state, and that the argon atom lies on the threefold symmetry axis on the oxygen end of methoxy.
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Introduction Over the past several years we have been exploring various ways of extending the technique of supersonic free-jet cooling to organic free radicals. As we demonstrated with methylene,' it is possible to generate intense jets of cooled radicals by photolyzing a stable precursor in the early stages of a pulsed jet expansion. Collisions with carrier gas in the subsequent portion of the expansion then cools the radical translationally, and (to some extent) cools the internal degrees of freedom as well. In the case of methylene a very substantial rotational cooling was noted but essentially no vibrational relaxation could be attained even at the most extreme expansion conditions. This is not too surprising in this special case since all vibrations in methylene are over 1500 cm-l and the vibrational level density is extremely sparse even at high energies. Larger radicals would be expected to show more effective vibrational cooling in such an environment, but the photolytic process is likely to deposit a large fraction of the available excess energy in the vibrational degrees of freedom of these larger radicals and the initial vibrational temperature could well be thousands of degrees Kelvin. Even in the most intense pulsed free-jet expansions we have used, these vibrationally super-hot radicals would not be completely cooled. To solve this vibrational cooling problem we have developed a simple pulsed nozzle which permits laser photolysis of the stable parent molecule in a high-pressure region just prior to free expansion. Sufficient time is available here to relax the new-born radicals to a roomtemperature vibrational distribution which is then easily cooled the rest of the way to -0 K by the free expansion. The nozzle used to perform this task is so simple and the cooling performance so complete that a wide range of large, complicated radicals should now be quite readily studied by this technique. The particular radical chosen to test this new technique is methoxy, an unusually interesting species in that it has a 2E ground state (C% point group). Ab initio calculations2 show the unpaired electron to lie in an almost pure oxygen 2p, orbital and predict the Jahn-Teller distortion to be extremely small. Russell and Radford3have published an extensive analysis of the laser magnetic (LMR) spectrum of methoxy in which they have determined to high precision a wide variety of constants (seventeen in all) de(1) I:1. L.Monts, T. G. dietz, M. A. Duncan, and R. E. Smalley, Chem. Phys., 415, 133 (1980). (2) 1. R. Yarkony, H. E. Schaefer, 111, and S. Rothenberg, J. Am. Chern. SOC.,96, 656 (1974). (3) (a) D. K. Russell and H. E. Radford, J. Chern. Phys., 72, 2750 (1980); (b) H. E. Radford and D. K. Russell, ibid., 66, 2222 (1977). 0022-3654/81/2085-2711$01.25/0
scribing the rotational and magnetic behavior of the 2 state. Among these constants are the rotational parameters A and B (5.328 and 0.9318 cm-l, respectively) which fit well the bond distance and angle calculations of Yarkony et ala2Also included in the LMR fit were the spinorbit coupling coefficient, 4 (-142.8 cm-'), and the component of electronic angular momentum about the C-0 bond axis, (0.44 h). While these constants were determined to an apparently high accuracy, the results of any such multiparameter least-squares fit may represent only a locally good choice in the multiparameter space and are, of course, sensitive to the model used. In order to provide an independent check of the Ryssell-Radford parameters, a rotational analysis of the A 2A1 2E ultraviolet spectrum4would be extremely useful. Unfortunately, the room-temperature absorption and emission vibronic bands are far too complex for ready interpretation by conventional techniques. In the cold jet, however, this complexity vanishes and assignment of the spectra discussed below for methoxy and its argon van der Waals complex is relatively straightforward.
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Experimental Section Figure 1shows a cross section of the pulsed photolysis nozzle used in this work. The key feature is a teflon disk (seen edge-on in Figure 1)which is clamped to the front of a pulsed valve where it provides a high-density gas region for laser photolysis prior to free-jet expansion. This photolysis region is simply a cylindrical channel 0.5 cm long, 0.2 cm diameter through which a laser-access port (also 0.2 cm diameter) has been drilled at right angle. Although gas in the channel is free to escape out either end of the laser-access port, the length of this port is sufficient that the bulk of the gas flow is straight down the channel to form the desired free-jet expansion with the end of the channel acting as the sonic orifice. There is, therefore, no need for windows on the ends of the laser-access port. In fact, blocking these ends causes increased turbulence in the main channel with the net result being an actual decrease in the flow through the sonic orifice. The pulsed valve used to feed gas to the teflon photolysis channel was a solenoid-actuated, commercial valve with a 0.07 cm diameter orifice (General Valve Corp., East Hanover, NJ, Model GVC-PIN-9). With a simple lowvoltage (150 to 200 V) power supply and a switching transitor (Motorola MJ12002), this valve can be driven at 10 Hz to produce flat-topped pulses with a rise time of 0.1-0.2 ms and a duration of approximately 1 ms. During (4) (a) G. Inoue, H. Akimoto, and M. Okuda, J. Chern. Phys., 72,1769 (1980); (b) G. Inoue, H. Akimoto, and M. Okuda, Chern. Phys. Lett., 63, 213 (1980).
@ 1981 American Chemical Society
2712
Letters
The Journal of Physical Chemistty, Vol. 85, No. 19, 1981
0 0, BAND
1 ' ~ ~ 1 " ' 1 ' " 1 1 " " 1 ~ 1 ' ' ' 1 '
100
150
50
0
PHOTOLYSI s LASER
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RELATIVE FREQUENCY
100
( C d '1
Figure 2. Laser-induced fluorescence excitation spectrum of the jet-cooled methoxy radical in the region of the 'A1 A E ,', transition origin near 31 540 cm-'. The cluster of strong peaks spanning 15 cm-' around the zero of the frequency scale is due to transitions from the J = 0.5, K = 0 and J = 1.5, K = 1 rotational levels of the 'E3/* state of methoxy. The two strong peaks centered about -23 cm-' on the frequency scale are due to corresponding rotational levels of the argon-methoxy van der Waals complex. The sharp single features at 52 and 96 cm-' are believed due to impurities. Photolysis nozzle conditions: 2.4-atm argon backing pressure, Nd:YAG laser fourth harmonic photolysis of methyl benzoate (at 20 'C vapor pressure). +
Figure 1. Cross-sectional view of pulsed nozzle used to produce cold methoxy radicals by laser photolysis. The photolysis occurs inside a 0.2-cm diameter teflon channel which is pressurized with a dilute mixture of methyl benzoate in argon or helium flowing from a pulsed valve. The photolysis laser is focused into this channel through a 0.2-cm diameter access port.
the flat-topped region of the pulse, the gas flow was measured to be 170% of the full-open value expected for a 0.07-cm diameter orifice. This high flow rate and the ability to produce such a flow repeatably a t high backing pressures were crucial to the successful operation of the teflon laser photolysis channel shown in Figure 1. Methyl benzoate was selected as the stable photolytic precursor of methoxy since it was readily available and has broad, strong absorptions in the ultraviolet both at the 1930-A wavelength of the ArF excimer laser, and at 2660 A for the Nd:YAG fourth harmonic. The room-temperature vapor pressure (-0.5 torr) produced a sufficient concentration in the helium (or argon) carrier gas for the spectral work reported below. The 10-mJ photolysis laser beam was focused through the access port in the teflon nozzle, forming a waist of -0.1 cm diameter in the photolysis channel. This photolysis laser was synchronized to fire just after thtgas density had reached maximum in the channel. The A 2A1 X 2E absorption spectrum of the resultant jet-cooled methoxy radicals was then probed by laser fluorescence excitation at a point 10 cm downstream of the nozzle exit.
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Results and Discussion Figure 2 displays the fluorescence excitation spectrum in the region of the 0; band for methoxy cooled in an argon jet. A t a backing pressure, Po, of 1 atm or less the pair of features centered around -23 cm-l is not seen. This pair grows in with increasing argon pressure and is attributed to the Ar-OCH3 van der Waals complex. Argon clustering was found to be so facile with this new nozzle design that operation with Po greater than 5 atm was prohibited. At such a high argon pressure, most of the methoxy radicals become involved in large argon clusters and the sharp spectrum seen in Figure 2 is replaced by a large, continuous baseling signal. The A 2Al excited state of methoxy is formed by promoting a u-bonding electron (involved in the C-0 bond) into the unfilled 2p, orbital on the oxygen.2 As a result the C-0 bond strength is markedly reduced and the equilibrium bond length increases froF its origLnal value of -1.44 A to, roughly, 1.65 A. The A 2A1 X 2E excitation spectrum is therefore dominated by a long progression in the C-0 stretching mode, 3, which is 1015 cm-l
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0
RELATIVE FREQUENCY
50 (crn-')
Figure 3. Methoxy fluorescence excitation spectrum in region of 3: band of the 2Al absorption spectrum. The three major vlbronic bands seen in this re ion are believed due to a Fermi resonance between the 3', 3' 2 , and 2' vibronic levels of OCH,. Note the features due to ArOCH, red-shifted 26 cm-' from the correspondlng OCH, bands in the scan done with argon carrler gas. Note also the extra rotational transition seen on the high-frequency side of each methoxy band in the scan done with helium carrier gas. This Is due to a transition from the ( - I ) rotational level J = 0.5, K = 1 which is difficult to cool with helium. Nozzle condltlons: 2 atm argon (4.4 atm helium), 10 mJ photolysis laser (similar resutts were seen with both ArF excimer laser or fourth harmonic YAG photolysis). +-
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in the 2 state, reduced to 671 cm-l in the A state.4 Figure 3 presents the jet-cooled spectrum in the region of the 3; band of methoxy. Surprisingly, there are two major bands here split by 21 cm-l, and a third, somewhat weaker, band split off 39 cm-I to higher frequency. Similar scans of the spectrum in the vicinity of the 3; and 3: bands have also shown a splitting into two major, roughly equal intensity bands separated by 38 and 30 cm-I, respectively. Since this splitting is not seen at the origin (see Figure 2), it must occur in the A 2A1state and is quite likely due to a Fermi
The Journal of Physical Chemistry, Vol. 85, No. 19, 1981 2713
Letters
‘T ;F/
4 4-
/
G
3-
” 2\
2’57
I .5 0.5
1
K= 0
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5
K= I
1(+,
K=2
Figure 4. Rotational level diagram for the 2 and A ‘A1 states of methoxy. Rovlbronic transitions are shown relevant to the jet-cooled methoxy spectrum. The transitions A-F originate from the lowest rotational levels of each of the two unique nuclear spin symmetries. These transitions explain the entire observed rovlbronic band structure seen for methoxy cooled in an argon jet. The one transition (G) shown from a “hot” rotational level (a (-4 level with K = 1, J = 0.5) accounts for the extra transition observed in the band structure of methoxy- when helium expansion gas is used. The numbers listed for each X 2E3,2 level are the relevant values of J , the total angular momentum. The sign in parentheses In the K = 1 stack refers to the sign of the coupling between the electronic angular momentum, le,and the component of rotational angular momentum along the symmetric top axis. The numbers llsted by each level in the A 2A1 state refers to the value of N, the total angular momentum excluding spin. Each such level is really a neardegenerate doublet with J = N f 0.5 (except N = 0 which has only the level J = 0.5).
resonance involving the CO stretching mode, 3, and the CH3umbrella mode, 2, which is in the range of 1300-1400 cm-’ in similar molecules such as CH3Cl.e One therefore expects a resonance between 32 and 2l, between 33 and 2l3l, and among the levels 34, 32 2l, and 22. This is consistent with the observed number of strong bands in these three regions of the spectrum. The rotational structure seen in these cold vibronic bands of methoxy is quite simple; there are five major features with the largest actually being a doublet. Figure 4 shows the assignment of these six rovibronic transitions. They arise from AK = k l transitions originating in the lowest rotational level of _each of the two distinct nuclear spin symmetries. The X state energy levels shown in Figure 4 were calculated by using the constants and model of Russell and R a d f ~ r d .The ~ A state levels shown in the figure were calculated for the 2A1geometry predicted by Yarkony, neglecting spin-rotation coupling. The calculated transitions (A-F in Figure 4) using these energy levels agree perfectly with the observed rotational structure. Although (5) H. R. Wendt and H. E. Hunziker, J. Chem. Phys., 71,5202(1979). (6)G.Herzberg, “Molecular Spectra and Molecular Structure”, Vol. 11, Van Nostrand-Reinhold, New York, 1945, p 315.
the resolution of the experimental spectra is insufficient to test the high precision of the Russell-Radford constants, the observed spacing between the p-form (AK = -1) and r-form (AK= +1) subbands is a sensitive measure of fe, the electronic angular momentum about the CO bond axis. For le = 0, this splitting would be (A’ + A” - B’ - B”) = 9 cm-l, whereas for le= 1 the splitting would reduce to (A’ - A” + B’ - B”) = 0.2 cm-’. The observed splitting for the 0: band in Figure 2 is 6.3 f 0.5 cm-l. The rotational structure of the methoxy-argon complex is also simple. As seen in Figures 2 and 3, there are just two broadened features resolved in each vibronic band, split by 5.7 f 0.5 cm-l. This band contour is consistent , axis, only with a model where the argon is bound on the C leaving the A rotational constant and the electronic angular momentum largely unaffected. The two features are then simply the r-form and p-form subbands. Since the lowest unoccupied molecular orbital of methoxy is the u* antibonding CO orbital which extends well out from the oxygen on the C-0 axis,2 the argon atom acting as a Lewis base would be expected to bond at this site,’ retaining overall CSusymmetry. Figure 3 also shows the jet-cooled spectrum of methoxy in the 3: regjon when helium is used as the expansion gas, Here there is a new rotational feature seen which is remarkably difficult to cool. Although this particular scan was done at a rather modest backing pressure of 4.4 atm, we have performed scans of this spectral region using backing pressures of helium up to 22 atm and still this “hot” rotational feature has been cooled to only half the intensity seen in Figure 3. As shown in Figure 4, this hot feature fits excellently with G, the only transition allowed from the level J = 0.5, K = 1. This level has the correct nuclear spin symmetry to relax into the K = 0 manifold but cannot relax further in the K = 1 manifold. It is the lowest level of its particular nuclear spin symmetry where the electronic angular momentum and rotational angular momentum (due to tumbling of the nuclear framework) add with opposite sign to form the net angular momentum, K , along the top axis. The corresponding level J = 1.5, K = 2 which is the lowest (-1) level with the other nuclear spin symmetry is apparently quite easy to cool; we see no sign of transitions from this level even at low helium pressures, Po. This may be due to the near coincidence of the (+1) level J = 5.5, K = 1 which is calculated to lie only 0.7 cm-’ to higher energy.3
Acknowledgment. We are indebted to R. P. Mariella of Allied Chemical Corp. for valuable discussions and for calling to our attention the General Valve Corp. pulsed nozzle so vital to this experiment. R. F. Curl originally suggested the methoxy problem as one where jet cooling would be useful. We have benefited much from his help and guidance. This research was supported by the Department of Energy, Division of Chemical Sciences, and by The Robert A. Welch Foundation. ~~
(7) (a) S.J. Harris, K. C. Janda, S. E. Novick, and W. Klemperer, J. Chem. Phys., 63,881 (1975); (b) K. C.Janda, L. S. Bemstein, J. M. Steed, S. E. Novick, and W. Klemperer, J.Am. Chem. SOC.,100,8074 (1978); (c) K. C.Janda, J. M. Steed, S. E. Novick, and W. Klemperer, J. Chem. Phys., 67,5162 (1977).
