Rotational energy transfer in highly vibrationally excited HCN

Abstract

The state-resolved collisional self-relaxation of HCN at a vibrational energy content of 10 000 cm -1 is probed directly by combining direct overtone vibration excitation, to prepare energized molecules in the (0,0°,3) levei, with a laser induced fluorescence monitor of the population evolution from different rotational states. Pure rotational energy transfer dominates the collision dynamics while vibrational relaxation results from only a small fraction of the inelastic events. The depopulation of single j levels proceeds with high efficiency. It is characterized by rates up to 14 times faster than the Lennard-Jpnes gas kinetic rate conforming to a j dependent distribution which peaks near the Boltzman population maximum and decreases to higher and lower angular momentum valu~s; Approximately 70% of the collisional population removal from the j=41evel proceeds via the ilJ=±1 channel and 28% proceeds via the I:l.J=±2 direct population transfer step. The results support a long range dipole-dipole mechanism for the energy transfer. This work also investigated various empirical scaling relations and determined that a two parameter fitting law based on the momentum gap or a three parameter modified scaling expression based on the energy gap successfully models the rotational relaxation.

Line-mixing in the 106←000 overtone transition of HCN

“…Therefore, our data would indicate that a value of F in the range 0.5-0.7 is caused by dephasing processes other than e -f changing collisions. This is also consistent with the results of Wu et al, 18 who directly measured some of the collisional population transfer rates in HCN. They found that population transfer accounts for only ϳ70% of the pressure broadening rate, while the remaining 30% should correspond to optical collisional dephasing.…”

“…From these graphics, it is clear that in the MEG model there is a strong preference for collisions that change J only by Ϯ1. This is consistent with dipoledipole scattering in the first order Born approximation, and also with the recent experimental work of Wu et al, 18 who observed about 70% of population transfer to be due to ⌬JϭϮ1 collisions. On the other hand, the fitted ECS model gives significant rates for larger ⌬J.…”

“…Thus, it appear that no definite conclusions can be drawn about the degree of coherence transfer in HCN self-collisions without comparison with measurement or calculation of the state to state population transfer rates. The limited state to state population relaxation measurements reported by Wu et al 18 are in agreement with the MEG rate law prediction that the preponderance of collisions change J by at most Ϯ2. Further, the observed total rate of depopulation was found to be only 70% of that predicted from the measured pressure broadening coefficients, assuming negligible dephasing.…”

“…This argument is clearly not rigorous, and it is often the case that empirical laws are not satisfactory when extrapolated to compare with new data. In the present case, it is clear that J-resolved state-to-state rates, as recently measured by Wu et al for a limited range of J values, 18 would add important new information, and may allow one to choose the ''right'' fitting law in a direct way.…”

State-resolved collisional energy transfer in highly excited NO2. I. Cross sections and propensities for J, K, and mJ changing collisions

“…This results mostly from experimental difficulties in the state-selective preparation and the detection of the energized molecules. However, recent laser based double resonance type experiments on C 2 H 2 , [9][10][11][12][13] HCN,14 NO 2 , 15 and H 2 CO, [16][17][18] which directly monitored the energy and population redistribution within the highly excited polyatomic molecules, provided a first picture of collisional energy transfer at energies above 10 000 cm Ϫ1 . In contrast, at low internal energies, detailed studies are much more common, leading to a consistent physical picture of inelastic processes in this regime.…”