An experimental study of OH(A2Σ+) + H2: Electronic quenching, rotational energy transfer, and collisional depolarization

Abstract

Zeeman quantum beat spectroscopy has been used to determine the thermal (300 K) rate constants for electronic quenching, rotational energy transfer, and collisional depolarization of OH(AΣ) by H. Cross sections for both the collisional disorientation and collisional disalignment of the angular momentum in the OH(AΣ) radical are reported. The experimental results for OH(AΣ) + H are compared to previous work on the OH(AΣ) + He and Ar systems. Further comparisons are also made to the OH(AΣ) + Kr system, which has been shown to display significant non-adiabatic dynamics. The OH(AΣ) + H experimental data reveal that collisions that survive the electronic quenching process are highly depolarizing, reflecting the deep potential energy wells that exist on the excited electronic state surface.

Full-dimensional quantum stereodynamics of the non-adiabatic quenching of OH(A2Σ+) by H2

“…As discussed in the next section, we attribute the experiment–theory discrepancy to the large yield of the adiabatic elastic and inelastic channel (channel 3), which was neglected in the previous analysis of the branching ratio 28 . To this respect, the recent work of Brouard et al 34 indicated that the inelastic scattering cross section is three times as large as the quenching one (channel 1 + channel 2) for the ground rotational state of the OH(A) reactant ( N OH = 0), confirming our results.…”

“…Importantly, this particular steric approach has a large cone of acceptance leading to the large yield for channel 3 shown in Table 1 , although the dominance of this elastic and inelastic channel is weakened at higher energies. The adiabatic elastic and inelastic channel has been shown by Brouard et al to be quite dominant 34 .…”

“…Unfortunately, the existence of this elastic and inelastic channel was not considered in their branching ratio model. The recent experiment of Brouard et al 34 found that quenching (reactive and non-reactive) represents only a minority of the scattering outcome, with a cross section that is only one-third of that for inelastic scattering. When elastic scattering is included, the adiabatic elastic and inelastic channel would have an even larger cross section.…”

“…When elastic scattering is included, the adiabatic elastic and inelastic channel would have an even larger cross section. The experiment of Brouard et al 34 was performed under thermal conditions (~0.039 eV), which is close to the collision energy of 0.05 eV. Indeed, if we subtract the calculated fraction of channel 3 ( f 3 = 0.772) from the experimental value for channels 1 and 3 (0.875), the fraction of channel 1 ( f 1 ) would be 0.103, which is very close to our calculated value of 0.098.…”

“…Furthermore, the Lester group reported that the branching ratio between the reactive and non-reactive quenching channels favours the former 28 . However, more recently, Brouard and co-workers determined the cross sections of the inelastic channel and the total (reactive plus non-reactive) quenching in absolute units, finding that the inelastic channel was three times larger than that of the total quenching channel 34 . The existence of quantum state-resolved experimental data makes this system a fertile proving ground for theoretical investigations.…”