J . Phys. Chem. 1994, 98, 9285-9290
9285
Infrared-Ultraviolet Double-Resonance Measurements on the Temperature Dependence of Relaxation from Specific Rovibronic Levels in NO(X211, v = 2, J) and (X*II, v = 3, J) Meezanul Islam, Ian W. M. Smith,' and Jorg W. Wiebrecht School of Chemistry, The University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K. Received: March 28, 1994"
Infrared-ultraviolet double-resonance experiments have been performed on N O a t three temperatures, 295, 200, and 80 K, to measure rate constants for (a) total relaxation from selected levels in the u = 2, Q = l / 2 and u = 3, Q = l / rotational ~ manifolds of the X211 electronic ground state with several collision partners (M = N O , He, Ar, H2, Nz, CO, and COz), and (b) vibrational self-relaxation from u = 2 and u = 3. N O molecules were initially prepared in selected rovibronic levels by tuning the output from an optical parametric oscillator to lines in the (2,O) or (3,O) infrared overtone bands. Loss of population from the initially excited level was observed by making time-resolved laser-induced fluorescence measurements on appropriate lines in the (2,2) and (2,3) bands of the A22+-X211 electronic system of NO. The thermally averaged cross sections for total rotational relaxation are found to be essentially independent of rotational state and temperature. The light collision partners (He, Hz) are the least effective, with the molecular species (NO, N2, CO, and C02) rather more effective than Ar. The results are compared with previous directly determined values for rotational relaxation in u = 2 and higher vibrational levels and with cross sections inferred from measurements of linebroadening. It is clear that vibrational self-relaxation of NO(u=2) and NO(u=3) occurs by vibration-vibration (V-V) exchange, NO(u) NO(u=O) NO(u-1) N O ( u = l ) , a t a rate which is almost independent of temperature and which seems to be uninfluenced by the presence of spin-orbit degeneracy in, and specific attractive forces between, the N O collision partners.
+
-
I. Introduction The existence of the "stable" free-radical nitric oxide presents experimentalists with a unique opportunity to examine the characteristics of collisions involving an open-shell species with electronic angular momentum, without the added complication of creating that species in situ, for example by photodissociation of a precursor molecule. Furthermore, with a 2 I I species, such as N O in its electronic ground state, collisions can change the spin-orbit and A-doublet, as well as thevibrational and rotational, states of the molecule. It is important to establish the cross sections and/or rate coefficients for all these inelastic processes and how they depend, inter alia, on the initial state, the collision partner, and the collision energy or temperature. A comparison of the results of such experiments, either with the results of scattering calculations on accurate or model potentials122 or with the results of similar experiments on closed-shell species,2 can then reveal what parts of the intermolecular potential induce the various changes in internal quantum state which are brought about by collision. It is instructive to consider the vibrational relaxation of N O as an example. The self-relaxation of NO(v=l)3 and the relaxation of NO(v=l) by closed-shell species such as the rare gases must occur by vibration-rotation, translation (V-R,T) energy transfer394 and is known to be facile, relative to, for example, the relaxation of C O ( U = ~ )which , ~ has a similar vibrational frequency and reduced mass, but a IZ+electronic state. This difference is attributed6 to an electronically nonadiabatic mechanism with N O which can occur because of the existence of multiple potential energy surfaces. On the other hand, the vibrationvibration (V-V) self-relaxation of higher vibrational states of NO, i.e. NO(u)
+ NO(u=O)
NO(u-1)
-+
+ NO(u=l)
(1)
occurs at r a t e s 3 ~ ~which - ~ are comparable to those found for selfrelaxation processes in CO with similar energy discrepancies."-'Jl Abstract published in Advance ACS Abstracts, August 15, 1994.
0022-3654/94/2098-9285%04.50/0
+
In NO, as in other diatomic and simple polyatomic molecules, rotational energy transfer is much faster than vibrational relaxation. Rotational relaxation can occur within the same spinorbit state in R-conserving collisions or between spin-orbit states in R-changing collisions. Such processes have been studied recently and quite extensively in both crossed-beam'2-14 and gas ce117,9J5J6experiments. Each type of measurement has its advantages and limitations. In the beam measurementsreported so far, initial state selection has been achieved by supersonic expansion of a mixture of NO in an excess of Ar carrier. In this way, more than 90% of the NO molecules are prepared in the i? = l/2, u = 0, J = I / * state, with very nearly all the remainder in i? = I/2, v = 0, J = 3/2. This beam has been crossed with a pure argon beam at a collision energy corresponding to 444 cm-I, and the relative populations of the states to which N O molecules were transferred were determined by laser-induced fluorescence (LIF) spectroscopy or resonance-enhanced multiphoton ionization (REMPI) with direct ion imaging. These elegant experiments are capable of providing exquisite detail: differentialstate-to-state cross-sections for energy transfer at a defined collision energy under single collision conditions; but they are, as presently performed, restricted in terms of the initially selected state. Furthermore, their sophistication and the sheer amount of the detail they provide means that, at least so far, the results are confined to a restricted number of collision partners and a single value of the collision energy. In gas cell experiments, the state-resolved details of energy transfer processes are most fully revealed in double-resonance (DR) experiments. In experiments of this kind, the Boltzmann distribution of molecules over rovibronic states is perturbed by pumping a particular spectroscopic transition using a powerful pulsed laser source and the rates and pathways of the relaxation processes are examined using spectroscopictechniques, most often, as in crossed-beam experiments on NO, using the LIF technique. One can determine the total rate of removal of molecules from the initially prepared state by scanning the time delay between the fixed frequency pulses from the excitation and probe sources, the output from the second laser being tuned to an ultraviolet 0 1994 American Chemical Society
9286
The Journal of Physical Chemistry, Vol. 98, No. 37, 1994
transition from the initially prepared state. Alternatively, the time delay is fixed and the frequency of the probe laser is scanned. With the delay chosen to be much shorter than the mean time between collisions, the LIF spectrum which is obtained reflects the relative concentrations in levels populated directly from the initial state in single collisions. Sudbo and LoyI5 were the first to apply an IRUVDR technique to NO. They excited specific (R,J) levels in NO(u=2) using a pulsed F-center laser and used a (1+1) REMPI technique todeterminestate-to-stateinformation about self-relaxation processes in NO. More recently, in a variant of the double resonance technique, Wodtke and co-workers7J6 have obtained rotational and vibrational relaxation rates within and for high vibrational levels of N O (8 Iu I24). Selected levels in the XzII ground state were populated by stimulated emission pumping (SEP) via levels in the B2II excited electronic state and redistribution of the excited population in the X211 state was observed via LIF spectroscopy. Perhaps the principal advantage of gas cell experiments over those in crossed molecular beams is that one can obtain overall and state-to-state rate coefficients for energy transfer from a wide range of initial rovibronic states. In addition, it is straightforward to change both the temperature of the sample and the nature of the collision partner. In a previous paper from our laboratory,g overall rate coefficients were reported for vibrational and rotational self-relaxation of NO at 295,200, and 80 K from three rotational levels in the X211,v = 2, R = ‘/2 state. In the present paper, we report the extension of the measurements to levels in u = 3 and to a wider range of collision partners; specifically He, Ar, H2, N2, CO, and C02. In these experiments, as in the earlier ones,g N O molecules are excited by pulses of radiation from a LiNb03-based optical parametric oscillator (OPO) and observed by LIFexcited by tuning a pulsed, frequencydoubled, dye laser to a suitable transition in the A22+-X211 electronic band system. The experiments which are reported in the present paper are to be followed by measurements of stateto-state rate constants for energy-transfer processes involving more limited ranges of initial state, collision partner, and temperature.17 We also report here rate constants for vibrational self-relaxation of NO(u=2) and NO(v=3) at 295, 200, and 80 K.
Islam et al. Model 9781B), through a U G l l broad-band filter which effectively cuts out radiation of wavelengths 10%. Table 1 includes rate constants for self-relaxation which are slightly different from those reported by Frost et al.9 Small errors were found in some of our earlier measurements, apparently because data were gathered a t probe laser powers which led to partial saturation of the LIF signals. Tables 1 and 2 also list the thermally averaged collision crosssections for removal of NO from specific levels. They arederived by dividing the second-order rate constants for total removal by the mean relative speed for collisions between N O and the collision partner (M). The resultant cross-sections for total removal of N O from the (v=2, !I= l/2, J = 13/2)by M = NO, He, Ar, and H2 are plotted against temperature in Figure 3. There appears to be no significant variation of the rate constants or cross-sections for total rotational relaxation on either the initial vibrational or rotational state of the N O molecule. Any dependence of the rate constants on temperature appears to follow the T1IZ dependence of the mean relative speed of the collision partners quite well; that is, the thermally averaged cross-sections exhibit little or no dependence on temperature, with the possible exception of those for self-relaxation. In this case, it does appear as if there is a small but significant increase in the thermally averaged cross-sections at the lowest temperature (80 K) of our measurements. This increase may be due to the stronger and specific attractive forces that are present in NO-NO collisions and which are responsible for the formation of the (N0)2 dimer at low temperatures.20 There have been very few direct measurements of rotational energy transfer in N O induced in thermal collisions with partners (M) other than N O itself. Sudbo and Loyls made limited measurements at 297 K on NO(v=2, R=I/Z, J) with M = Ar, He, N2, CO, and SF6. The thermally averaged cross-sections derived from their measurements on NO(v=2, !I=~/z, J=21/2)with He and NO(v32, !I=l/2, J=23/z) with Ar are shown in Figure 3. Their NO-He result is in excellent agreement with our data, assuming no dependence on J, their cross-sections for NO-Ar are slightly higher than our values. They also reported rate constants for total relaxation from NO(vr2, !I=l/2, J=21/2)with M = N2 and CO, corresponding to cross-sections of 58.4 f 9.2 and 55.3 f 4.6 A2, respectively, values whichare again in excellent agreement with our results for these systems. Rate constants for the total removal of population from a given rovibronic level can also be compared with line-broadening
TABLE 1: Rate Constants (k/10-lo cm3 molecule-1 s-1) and Cross-Sections (Az)for the Overall Removal of NO Molecules from Specific Rotational Levels in the Xzlll/z, v = 2 Vibronic Level in Collisions with Different Collision Partners at 295, 200, and 80 K 200 K
295 K
M NO He Ar
Hz Nz
CO COz
J = 0.5 3.56 f 0.3 55.2f4.6 2.17 f 0.3 16.3 f 2.2 1.93 f 0.4 31.9 f 6.6 3.50f0.4 19.1 f 2.2 4.02 f 0.4 60.2 f 6.0 3.93 f 0.4 58.8 f 6.0 3.73 f 0.5 70.0 f 9.4
J = 6.5 3.77 f 0 . 2 58.4f3.1 2.35 f 0 . 2 17.7 f 1.5 1.90 f 0.2 31.5 f 3.3 3.55 f 0 . 4 19.5 f 2.2 3.54 f 0.5 53.0 f 7.5 3.54 f 0.4 53.0 f 6.0 3.00 f 0.4 53.5 f 7.5
J = 15.5 3.43 f 0 . 2 53.lf3.1 2.51 0.5 18.8 f 3.8 2.11 f 0.2 34.9 f 3.3 3.42 f 0.3 18.7 f 1.6 3.96 f 0.4 59.3 f 6.0 3.70 f 0.4 55.4 f 6.0 3.61 f 0.2 67.8 f 3.8
*
J = 0.5 2.65 f 0.2 49.8f3.8 1.62f0.3 14.8 f 2.7 1.69 f 0.3 34.0 f 6.0 2.04f0.3 13.6 f 2.0
J = 6.5 2.97 f 0 . 2 55.9*3.6 2.11 f 0.4 19.2 f 3.7 1.88 f 0.3 37.7 f 6.0 1.92 f 0 . 3 12.8 f 2.0
80 K J = 15.5 2.73 f 0.3 51.7f5.0 1.92f0.3 17.5 f 2.7 2.01 f 0.3 40.4 f 6.0 2.01 f 0 . 3 13.2 f 2.0
J = 0.5 2.17 f 0 . 2 64.6f4.4 1.08 f 0 . 2 15.6 f 2.9 1.34 f 0.2 42.6 f 6.4 1.46f0.3 15.4 f 3.2 2.48 f 0.2 71.3 f 5.8 2.51 f 0.3 72.2 f 8.6
J = 6.5
J = 15.5
2.36f0.2 70.4f6.0 1.47 f 0.3 21.2 f 4.3 1.13 f 0.2 35.3 f 6.2 1 . 3 0 f 0.4 13.6 f 4.2 2.33 f 0.2 67.0 f 5.8 2.12 f 0.3 61.0 f 8.6
2.67 f 0.3 79.5f8.9 1.28 f 0 . 3 18.5 f 4.3 1.76 f 0.3 55.9 f 9.5 1.53 f 0 . 3 16.1 f 3.2
The Journal of Physical Chemistry, Vol. 98, No. 37, 1994
9288
Islam et al.
TABLE 2 Rate Constants (k/10-lo cm3 molecule-l s-l) and Cross-sections (Az)for the Overall Removal of NO Molecules from Specific Rotational Levels in the X211t/2,
Y
= 3 Vibronic Level in Collisions with Different Collision Partners at 295, 200, and 80 K
295 K
J = 0.5 3.16f0.3 49.0 f 4.6 2.88 f 0.3 2 1 . 6 f 2.3 2.89 f 0.3 47.8f5.0 3.50f 0.4 19.2 f 2.2 3.59 f 0.3 67.4 f 5.6
M NO
He Ar H2
CO2
J = 4.5 3.24f0.3 50.2 f 4.6 2.33 f 0 . 3 17.5 f 2.3 2.23 f 0.3 36.9f5.0 3.07 f 0.3 16.8 f 1.6 3.63 f 0.4 68.1 f 7.5
200 K
J = 8.5 3.43 f 0.4 53.1 f 6.2 2.2 f 0.3 16.7 f 2.3 2.92 f 0.4 48.3f6.6 3.28 f 0.4 18.0 f 2.2 3.25 f 0.3 61.0 f 5.6
J = 0.5 2.8 f 0 . 8 52 f 14 2.40f 0.8 21.8f 7.0 2.45 f 0.3 49.2f6.0 1.92 f 0.4 12.8 f 2.7
80 K
J = 8.5 3.00f 0.8 56.3f 15 2.41 f 0.8 22.0 f 7.3 2.24 f 0.3 45.0f6.0 2.08 f 0.4 13.8 f 2.7
J = 4.5 3.0f0.8 5 6 f 15 1.7 f O . 5 15 f 4 2.79 f 0.3 56.1f6.0 1.89 f 0.3 L2.6 f 2.0
100 2*o
k
J = 4.5
J = 8.5
2.09 f0.4 6 2 . 2 f 11.9 1.21 f 0 . 4 17.5 f 5.7 1.13 f 0.3 35.9f9.5 1.11 f 0.3 11.7 f 3.2
2.63 f 0 . 4 78.3f 11.9 1.18 f 0.3 17.0 f 4.3 1.33 f 0.3 42.3f9.5 1.51 f 0.4 15.9 f 4.2
1.91 f 0.3 56.8f8.9 1.01 f 0.4 14.6 f 5.8 1.64 f 0.4 52.1f12.7 1.59 f 0.4 16.7 f 4.2
I
-'--
N
o
=b
J = 0.5
PI-
0
50
1.0
-
v
Y
0.0"
"
"
"
"
'
I
"
"
"
LI
0 100
200
300
Temperature/K
Figure 3. Thermally averaged cross-sections for total relaxation of NO( ~ 2C ,l = l / ~ , J=I3/2) by NO ),(. Ar (A),He (V),and Hz).( at different temperatures. The open symbols represent the results of Sudboand LoyI5 (see text for details). transitions terminating on the samestate. For rovibrational levels in the electronic ground states of C2H2lSaand N2,z1 for example, rate constants derived from the two kinds of measurement correlate well, indicating that energy transfer rather mrchanging collisions, are predominantly responsible for collision-broadening in these molecules. A large number of studies have been carried out on the pressurebroadening of infrared transitions in NO.2Z Two of the most recent and thorough are those of Pine et al.23on lines in the (2,O) first overtone band. Their papers report collision-broadening coefficients at room temperature for NO, Nz, and Ar as broadening gases for a number of (mainly R-branch) lines in the two subbands, R = R = '/z and R = 3 / 2 52 = 3/2, of the (2,O) overtone transition. Second-order rate constants can be calculated from collision-broadening coefficients and then compared with those for rotational energy transfer determined in experiments like those reported in the present paper. Any discrepancy indicates the existence of collision-broadening mechanisms other than rotational energy transfer. As noted previously,g it appears that, in NO-NO collisions, there is a small but significant discrepancy between the two sets of data, suggesting that some other process does contribute to line broadening. As pointed out by Frost et al.,9 calculations by Orlikowski and Alexanderlb suggest that NO-NO collisions may be unusual in that the usual propensity for the conservation of mJmay be relaxed because of the 211 nature of both collision partners, so that ms changing collisions may contribute to pressure-broadening in pure NO. Figure 4 shows, for the case of NO-Nz collisions, a comparison of the rate constants derived from collision-broadening coefficients +
+
with those for rotational energy transfer which we have determined. Here, and in the similar case of NO-Ar, any difference between the directly measured rate constants and those inferred from line-broadening measurements is certainly less, and on the borderlineof being significant. It certainly appears that rotational energy transfer is at least the predominant process leading to broadening of lines in N O rovibrational transitions in the case where N2 or Ar is the collision partner. The thermally averaged cross-sections for rotational relaxation by different M show differences which are greater than those simply associated with the Lennard-Jones collision cross sections. They increase in sequence from M = He and H2, through Ar, to the molecular gases which we have studied as collision partners (NO, N2, CO, and C02). The relative inefficiency of H e and H2 in inducing rotational relaxation has been noted by Frost and SmithlSain relation to their study of rotational energy transfer in C2H2. They suggested that it might be caused by angular momentum constraints resulting from the comparatively small range of orbital angular momentum in thermal collisions with a small reduced mass. The proposition that it is factors involving angular momentum, rather than those associated with changes in rotational energy, that control probabilities of collision-induced rotational transfer has been developed very recently by McCafferty et al.24 We are currently undertaking extensive measurements of state-to-state rate constants for rotational transfer in NO, partly in an attempt to find further evidence for the controlling natureof angular momentum in rotationally inelastic collisions.17 Finally, we note that the present measurements appear to be the first to provide an extensive body of data about rotational energy transfer tolow temperatures. It is important tounderstand such processes a t ultralow temperatures in order to interpret astrophysical measurements on dense interstellar clouds. In the
The Journal of Physical Chemistry, Vol. 98, No. 37, 1994 9289
Temperature Dependence of Relaxation in N O
-2 e
-
10
H il >
v O
-3
V U
0
Q
m
V
0 -
V
V
-4
V
0
20
0
40
[N0]/10
16
20
0
m o l e c u l e .cm
spin-orbit populations have become thermalized, plotted against the concentration of NO. The insertsshow typical traces of LIFsignal against time from a single experiment on each of these vibrational levels.
TABLE 3 Rate Constants (k/10-l2 cm3 molecule-’ s-l) and Thermally Averaged Cross-Sections (AZ) for the Vibrational Self-Relaxation of NO(v = 2) and NO(v = 3) Molecules at 295,200, and 80 K
200 77
NO(v = 2)” 2.44f0.3 0.38 f 0.05 1.60f 0.3 0.30 f 0.06 2.10f0.2 0.77 f 0.07
2 . 8 0 f 0.34 0.44 f 0.05 1.96f 0.37 0.37 i 0.07 3.48f0.33 1.28 f 0.12
-5 0
100
200
300
(AE/hc)/cm-’ Figure 6. Comparison of reduced probabilities for V-V exchange and
their variation with energy discrepancy of CO-CO (v)and CO-NO (0) collisions1° with those for NO-NO collisions from the present work ( 0 ) and from ref 8 (0)
NO(v = 3)“ 2.22f0.2 0.34 i 0.03 2.50f 0.3 0.47 i 0.06 2.19f0.4 0.81 f 0.15
2.92f0.26 0.45 f 0.04 3.29f 0.40 0.62 i 0.08 6.02f1.1 2.23 f 0.41
The figures to the left are for the observed relaxation of NO@) by NO. Those to the right are for the exothermic V-V process, NO(v-1) + NO(v=l) NO@) + NO(v=O), calculated by applying detailed
-
balance on the assumption that the observed relaxation occurs entirely by endothermic V-V energy exchange. future, we expect to apply the IRUVDR method in the environment provided in a CRESU (Cin6tique de R6action en Ecoulement Supersonique Uniforme) apparatus where temperatures close to 10 K can be achieved.25 (b) Vibrational Relaxation. In addition to measuring the rate constants for vibrational self-relaxation of NO(v=2) which were reported earlier,g we have also determined the rate constant for vibrational self-relaxation of NO(v=3). Both sets of experiments were carried out on samples of N O diluted to a constant total pressure of 10 Torr with argon, which caused rapid rotational relaxation of the excited population and decreased the rate at which it diffused out of the region illuminated by the probe laser. The effect of argon on the rate of N O vibrational relaxation was negligible. As vibrational relaxation of both NO(v=2) and NO( ~ 3 is) approximately 100 times slower than rotational relaxation, the concentrations of N O in these experiments was increased to between 3 X 10’6 and 30 X 10’6 molecules ~ m - ~ . The LIF signals in these experiments also fitted singleexponential decays accurately. Some plots of the pseudo-firstorder rate constants against [NO] are given in Figure 5. The derived second-order rate constants for relaxation of NO(v=2) and NO(v=3), and the corresponding thermally averaged crosssections are given in Table 3. Relaxation of NO(v=2) and NO(v=3) undoubtedly occurs by V-V energy-transfer processes such as NO(v)
V
-3
Figure 5. First-order rate constants for the decay of LIF signals at 295 K from (a) NO(v=2), and (b) NO(v=3), in which the rotational and
T IK 295
V
40
+ NO(v=O) -NO(v-l) + N O ( v = l )
(1)
which are slightly endothermic, (AE/hc) = 27.9 cm-1 for v = 2 and 55.9 cm-l for v = 3, on account of vibrational anharmonicity. Our result for NO(v=2) has been compared in ref 9 with thedata of Stephenson,3 Macdonald and Sopchyshyn,26 and Horiguchi
and T s u c h ~ j a . ~Their ’ measurements were all less direct than the present experiments but the agreement between our results and theirs is satisfactory. Only Horiguchi and Tsuchuja have derived a rate constant for self-relaxation of NO(vt3). Their experiments involved measuring the phase-shift between the N O overtone emission spectra following transfer of energy from Hg(’P1) and the excitation of the Hg atoms. By analyzing the self-relaxation of NO(vllO), they found NO(v=3) to relax about 40% faster than NO(2-2). Our measurements indicate that these two rates are approximately equal, but this difference is not significant in the light of the uncertainties in both sets of experiments. In comparing rate constants and collisional probabilities for V-V energy-exchange processes involving molecules in different excited states, it is customary to use the data for exchange in the exothermic direction and to allow for changes in the vibrational matrix elements. Thus, for processes such as ( l ) , the rate constant for the reverse, exothermic process is calculated from that for the observed endothermic process by application of the principle of detailed balance. The resultant rate constants and thermally averaged cross-sections for the exothermic V-V exchange processes derived from the present measurements are listed in Table 3. Allowance for changes in the matrix element for the v, v - 1 transition can then be made by dividing the probability for each exothermic process by the appropriate value of v, to yield the reducedprobabilities introduced by Hancock and Smithlo in their extensive study of V-V processes involving CO(410113). Hancock and Smith’s results indicated that, for any given pair of non-hydride collision partners, the logarithm of the reduced probabilities, (PY,,,-l/v), fall off approximately linearly with the energy discrepancy, Le., the difference between the two vibrational transition energies. Moreover, the absolute values of (Py,y--I/v), reflect the strength of the infrared activity of the two collision partners. Figure 6 shows a plot of data from Hancockand Smith’s paper for energy transfer in CO-CO and CO-NO collisions. On this diagram, we have included the reduced probabilities for NON O collisions a t room temperature derived from our own experiments and, for v = 8, from the work of Wodtke and coworkers.8 These data are clearly consistent with the results obtained by Hancock and Smith.10 The mild temperature dependence of the rate constants for process (1) with v = 2 and v = 3 is also similar to what has been found for CO-CO collisions in various s t ~ d i e s . ~ . ~ It - l *has been
9290
The Journal of Physical Chemistry, Vol. 98, No. 37, 1994
taken as evidence for V-V energy exchange being induced predominantly by long-range attractive f o r ~ e s . ~ ~ , ~ ~
IV. Summary A double-resonance method employing tunable infrared and ultraviolet lasers has been used to study energy transfer in collisions involving NO. An extensive body of rate constants are reported for total transfer of NO from selected rovibrational levels in the X2n electronic ground state induced by a variety of collision partners and at temperatures down to 80 K. The results confirm that these rate constants are essentially independent of rotational and vibrational level-at least over the range of rotational levels significantly populated a t room temperature and below. For most the collision partners studied, the thermally averaged crosssections for total transfer are independent of temperature within experimental error. They do depend on the collision partner, being smallest for the light gases, H e and Hz, and largest for the molecular collision partners NO, N2, CO, and COz. There is an indication that the cross-section for rotational relaxation in NONO collisions increases at the lowest temperature, which may be due to the relatively strong directional forces associated with formation of the (NO)2 dimer. Vibrational self-relaxation has also been studied, for both NO( v = 2 ) and NO(v=3). The results indicate that relaxation proceeds by V-V energy exchange, probably under the influence of the long-range forces arising from interaction between the asymmetric charge distributions on both molecules and their modulation by vibrational motion. There is no firm evidence that the presence of spin-orbit degeneracies and specific attractive forces, associated with dimer formation. influences these rates. Acknowledgment. We are grateful to SERC for support of this work and the CEC for the award of a Fellowship (J.W.W.) under the Human Capital and Mobility Programme. References and Notes (1). (a) Orlikowski, T.; Alexander, M. H. J . Chem. Phys. 1983,79,6006. (b) Orlikowski, T.; Alexander, M. H. J. Chem. Phys. 1984, 80, 4133. (c) Alexander, M. H. J. Chem. Phys. 1993, 99,7725. (2) Brunner, T. A,; Pritchard, D. Dynamicsof the ExcitedSrate; Lawley, K. P., Ed.; Wiley: New York, 1982; p 590. (3) (a) Stephenson, J. C.J. Chem. Phys. 1973,59,1523. (b) Stephenson, J. C. J. Chem. Phys. 1974,60, 4289. (4) (a) Kamimoto, G.; Matsui, H.; J . Chem. Phys. 1970,53, 3987. (b) Glanzer, K.; Troe, J.; J . Chem. Phys. 1975,63, 4352. Glanzer, K. Chem.
Islam et al. Phys. 1977, 22, 367. ( 5 ) (a) Miller, D. J.; Millikan, R. C. J. Chem. Phys. 1970,53,3384. (b) Green, W. H.; Hancock, J. Chem. Phys. 1973,59,5326. Drozdoski, W. S.; Young, R. M.; Bates, R. D., Jr.; Hancock, J. K. J. Chem. Phys. 1976, 65, 1542. (c) Allen, D. C.; Price, T. J.; Simpon, C. J. S. M. Chem. Phys. 1979, 41,449. Mariq, M. M.; Gregory, E. A.; Wickham-Jones, C. T.; Cartwright, D. J.; Simpson, C. S. J. M. Chem. Phys. 1983, 75, 347. (6) (a) Nikitin, E. E. Opt. Specrrosc. 1960,9,8. Nikitin, E. E.; Umanski, Ya. Faraday Discuss. Chem. SOC.1972, 53, 1. (7) Yang, X.;Price, J. M.; Mack, J. A.; Morgan, C. G.; Rogaski, C. A.; McGuire, D.; Kim, E. H.; Wodtke, A. M. J. Phys. Chem. 1993, 97, 3944. (8) (a) Yang, X.;Kim, E. H.; Wodtke, A. M. J. Chem. Phys. 1990,93, 4483. (b) Yang, X.;Kim, E. H.; Wodtke, A. M. J. Chem. Phys. 1992,96, 5111.
(9) Frost, M. J.; Islam, M.;Smith, I. W. M. Can.J. Chem. 1994,72,606. (10) (a) Hancock, G.; Smith, I. W. M. Chem. Phys. Lett. 1970, 8, 41. Hancock, G.; Smith, I. W. M. Appl. Opt. 1971, 10, 1827. (11) (a) Wittig, C.; Smith, I. W. M. J . Chem. SOC.,Faraday Trans. 2 1973, 69,939. (b) Powell, H. T. 1973,59, 4937. (12) (a) Andresen, P.; Joswig, H.; Pauly, H.; Schinke, R. J . Chem. Phys. 1982, 77, 2204. (b) Joswig, H.; Andresen, P.; Schinke, R. J. Chem. Phys. 1986,85, 1904.
(13) Jons, S. D.; Shirley, J. E.; Vonk, M. T.; Giese, C. F.; Gentry, W. R. J. Chem. Phys. 1992, 97, 7831. (14) Bontuyan,L.S.;Suits,A.G.;Houston,P. L.; Whitaker,B. J.J.Phys. Chem. 1993, 97,6342. (15) (a) Sudbo, Aa. S.;Loy, M. M. T. Chem. Phys. Lett. 1981,82, 135. (b) Sudbo, Aa. S.;Loy, M. M. T. J. Chem. Phys. 1982, 76, 3646. (16) Yang, X.;Wodtke, A. M. J . Chem. Phys. 1992, 96, 5123. (17) Islam, M.; Smith, I. W. M.; Wiebrecht, J. W., to be published. (18) (a) Frost, M. J.; Smith, I. W. M. Chem. Phys. Lett. 1992,191,574. (b) Frost, M. J. J. Chem. Phys. 1993, 98, 8572. (19) Sharkey, P.; Smith, I. W. M. J. Chem. Soc., Faraday Trans. 1993, 89, 631. (20) Casassa, M. P.; Stephenson, J. C.; King, D. S. J. Chem. Phys. 1988, 89, 1966. (21) Sitz, G. 0.;Farrow, R. L. J . Chem. Phys. 1990, 93, 7883. (22) Smith, M. A. H.; Rinsland, C. P.; Fridovich, B.; Rao, K. N. In Molecular Spectroscopy: Modern Research; Rao, K . N., Ed.; Academic Press: New York, 1985; Vol. 3, p 112. (23) (a) Pine, A. S.; Maki, A. G.; Chou, N.-Y. J . Mol. Specrrosc. 1985, 114, 132. (b) Pine, A. S.J. Chem. Phys. 1989, 91, 2002. (24) McCaffery, A. J.; Alwahabi, Z. T.; Osborne, M. A.; Williams, C. J. J. Chem. Phys. 1993, 98, 4586. (25) (a) Sims, I. R.; Queffelec, J.-L.; Defrance, A.; Rebrion-Rowe, C.; Travers, D.; Rowe, B.; Smith, I. W. M. J. Chem. Phys. 1992,97, 8798. (b) Sims, I. R.; Queffelec, J.-L.; Defrance, A.; Rebrion-Rowe, C.; Travers, D.; Bocherel, P.; Rowe, B. R.; Smith, I. W. M. J. Chem. Phys. 1994,100,4229. (26) Macdonald, R. G.; Sopchyshyn, F. C. Chem. Phys. 1985,94455. (27) Horiguchi, H.; Tsuchuja, S.Jpn. J. Appl. Phys. 1979, 18, 1207. (28) Yardley, J. T. IntroductiontoMolecularEnergy Transfer;Academic Press: New York, 1980. (29) (a) Sharma, R. D.; Brau, C. A. J. Chem. Phys. 1969,50,924. (b) Sharma, R. D. Phys. Rev. 1969, 177, 102.
