Determination of the pair correlation of OH rotational states from the 266 nm photolysis of H2O2 using velocity-aligned doppler spectroscopy

International Reviews in Physical Chemistry

Perkins

et al. 2014

Recent work on collisional depolarisation in electronically excited radicals is reviewed, with a focus on the NO(A) and OH(A) radicals in collisions with rare gas atoms. The mechanism of depolarisation, and its competition with other collisional processes, is investigated using a combination of experimental and theoretical methods. The present state of the field is also put into context with other, related work in similar areas.

Section: Quantum Beat Spectroscopymentioning

confidence: 99%

Collisional depolarisation in electronically excited radicals

International Reviews in Physical Chemistry

Perkins

et al. 2014

“…OH(X) was generated by pulsed 193 nm photodissociation of hydrogen peroxide. [50][51][52][53][54][55][56][57][58][59] H 2 O 2 was flowed in a 50:50 mixture with water through the reaction chamber at a constant partial pressure of ≤2 mTorr. Translationally thermalized, electronically excited OH(A) radicals were obtained at a fixed pump-probe laser delay of ∼10 μs by pulsed excitation of OH(X) using the A 2 + ← X 2 transition.…”

Section: E Experimentalmentioning

confidence: 99%

The collisional depolarization of OH(A 2Σ+) and NO(A 2Σ+) with Kr

The Journal of Chemical Physics

Chang

et al. 2014

Quantum beat spectroscopy has been used to measure rate coefficients at 300 K for collisional depolarization for NO(A (2)Σ(+)) and OH(A (2)Σ(+)) with krypton. Elastic depolarization rate coefficients have also been determined for OH(A) + Kr, and shown to make a much more significant contribution to the total depolarization rate than for NO(A) + Kr. While the experimental data for NO(A) + Kr are in excellent agreement with single surface quasiclassical trajectory (QCT) calculations carried out on the upper 2A(') potential energy surface, the equivalent QCT and quantum mechanical calculations cannot account for the experimental results for OH(A) + Kr collisions, particularly at low N. This disagreement is due to the presence of competing electronic quenching at low N, which requires a multi-surface, non-adiabatic treatment. Somewhat improved agreement with experiment is obtained by means of trajectory surface hopping calculations that include non-adiabatic coupling between the ground 1A(') and excited 2A(') states of OH(X/A) + Kr, although the theoretical depolarization cross sections still significantly overestimate those obtained experimentally.

“…The experimental procedures for determining depolarization cross sections from Zeeman quantum beat spectroscopy have been described previously 1,4 , and only a brief summary will be given here. Translationally excited, superthermal OH(X) was generated by pulsed 248 nm or 193 nm photodissociation of hydrogen peroxide [27][28][29][30][31][32][33][34][35][36] . The H 2 O 2 itself was flowed in a 50:50 mixture with water through the reaction chamber at a constant partial pressure of ≤2 mTorr.…”

Section: Generalmentioning

confidence: 99%

“…The OH(X) products from the 248 nm photodissociation of H 2 O 2 are born translationally excited 1,27,[29][30][31][32][33][34][35][36] , with a narrow velocity distribution centred near 3850 m s −1 for N ′′ = 9. At 193 nm 28,45 , the velocity of the N ′′ = 10 fragments is sharply centered around 4550 m s −1 .…”

Section: Translational Moderation Of Oh(a)mentioning

confidence: 99%

Collisional angular momentum depolarization of OH(A) and NO(A) by Ar: A comparison of mechanisms

The Journal of Chemical Physics

Chang

et al. 2011

This paper discusses the contrasting mechanisms of collisional angular momentum depolarization of OH(A(2)Σ(+)) and NO(A(2)Σ(+)) by Ar. New experimental results are presented for the collisional depolarization of OH(A) + Ar under both thermal and superthermal collision conditions, including cross sections for loss of both angular momentum orientation and alignment. Previous work on the two systems is summarized. It is shown that NO(A) + Ar depolarization is dominated by impulsive events in which the projection of the angular momentum, j, along the kinematic apse, a, is nearly conserved, and in which the majority of the trajectories can be described as "nearside." By contrast, at the relatively low collision energies sampled at 300 K, OH(A) + Ar depolarization is dominated by attractive collisions, which show a preponderance of "farside" trajectories. There is also evidence for very long-lived, complex type trajectories in which OH(A) and Ar orbit each other for several rotational periods prior to separation. Nevertheless, there is still a clear preference for conservation of the projection of j along the kinematic apse for both elastic and inelastic collisions. Experimental and theoretical results reveal that, as the collision energy is raised, the depolarization of OH(A) by Ar becomes more impulsive-like in nature.