Coherence effects in the polarization of Lyman-α fluorescence following photodissociation of H2 and D2

We have studied the complete Cl-atom molecular-frame photofragment angular momentum distributions from the photodissociation of Cl 2 and ICl in the 320-560 nm region using time-of-flight mass spectroscopy with laser detection. The experimental signals were analyzed using the polarization-parameter formalism described in the preceding paper. These experiments study three distinct cases. The first case is the 470 nm dissociation of Cl 2 through the B 3 ⌸ 0 ϩ u state accessed via a parallel transition, yielding Cl-atom photofragments with polarizations described by the single parameter a 0 (2) (ʈ)ϭϪ0.7Ϯ0.2. The second case is the 320 nm dissociation of Cl 2 through the C 1 ⌸ 1u state accessed via a perpendicular transition, yielding Cl-atom photofragments with polarizations described by the two parameters a 0 (2) (Ќ)ϭϪ0.50Ϯ0.10 and a 2 (2) (Ќ)ϭϪ0.32Ϯ0.06. The third case is the dissociation of ICl in the 490-560 nm region in which dissociative states of both parallel and perpendicular character are accessed. In this wavelength region, the polarizations of the resulting Cl-atom photofragments are completely described by the approximately constant incoherent parameters, a 0 (2) (ʈ)Ϸϩ0.4, a 0 (2) (Ќ)ϷϪ0.2, and a 2 (2) (Ќ)ϷϪ0.2, whereas the interference contributions to the polarization, the Im͓a 1 (1) (ʈ ,Ќ)͔ and Re͓a 1 (2) (ʈ ,Ќ)͔, oscillate sinusoidally with excitation wavelength in a fashion that is sensitive to the shapes of the dissociative surfaces.

Section: Introductionmentioning

confidence: 99%

Measurements of Cl-atom photofragment angular momentum distributions in the photodissociation of Cl2 and ICl

Rakitzis

Kandel

Alexander

et al. 1999

“…As ;{ result, quantum mechanical effects have often been neglected in recent angle-resolved studies, although these effects can be very important. Coherence effects in atomic alignment, for example, have been the subject of considerable work in angle-averaged studies in photodissociation [10][11][12][13][14][15] as well as in collision studies [16] in recent years, but in principle the coherence contribution cannot be isolated from incoherent contributions in these studies. A full appreciation of the opportunities for exploiting atomic orbital polarization to probe coherence phenomena in angle-resolved photochemistry studies is only now beginning to emerge [17,18].…”

mentioning

confidence: 99%

Coherence in polyatomic photodissociation: Aligned O(3P) from photodissociation of NO2 at 212.8 nm

Ahmed

Peterka

Bracker

et al. 1999

Strong orbital alignment is observed in the ground-state oxygen atom following photodissociation of NO2 at 212.8 nm using ion imaging. The imaging method allows for investigation of the angular distribution of this alignment, providing insight into the dynamics in the frame of the molecule. The results are analyzed using a rigorous quantum mechanical theory yielding alignment parameters having direct physical significance. This alignment is dominated by a strong incoherent parallel contribution. In addition, the results reveal direct evidence of coherence between parallel and perpendicular contributions to the excitation of a polyatomic molecule, showing that the electron cloud in the recoiling atom “remembers” the original molecular plane.

“…10,27,28 In contrast, the fluorescence of excited H-atom states from the photodissociation of H 2 have been studied extensively. [29][30][31][32][33][34] The main reason for the lack of studies on ground-state H-atom photofragments has been that spin-sensitive detection of H atoms has-until now-been achieved only by resolving the fine structure of the 2p ← 1s transition; [35][36][37] this requires that the Doppler spread of the H atoms be very small, which will occur either in collimated atomic beams or from samples having a translational temperature of less than about 80 K. This constraint does not allow H-atom spin polarization to be detected without some stringent form of velocity selection; one example is the detection of H and D atoms from HCl and DCl photodissociation, by Koplitz and co-workers, [37][38][39][40] using velocity-aligned Doppler spectroscopy ͑VADS͒, which allows selective excitation of the fine structure of H-atom photofragments traveling parallel to the probe beam. This spatial velocity selection is achieved by detecting only the H atoms that have traveled about 0.5 m from the photolysis region to the probe, after a long time delay.…”

Section: Introductionmentioning

confidence: 99%

Laser detection of spin-polarized hydrogen from HCl and HBr photodissociation: Comparison of H- and halogen-atom polarizations

Sofikitis

Rubio-Lago

Bougas

et al. 2008

Thermal HCl and HBr molecules were photodissociated using circularly polarized 193 nm light, and the speed-dependent spin polarization of the H-atom photofragments was measured using polarized fluorescence at 121.6 nm. Both polarization components, described by the a(0)(1)(perpendicular) and Re[a(1)(1)(parallel, perpendicular)] parameters which arise from incoherent and coherent dissociation mechanisms, are measured. The values of the a(0)(1)(perpendicular) parameter, for both HCl and HBr photodissociation, are within experimental error of the predictions of both ab initio calculations and of previous measurements of the polarization of the halide cofragments. The experimental and ab initio theoretical values of the Re[a(1)(1)(parallel, perpendicular)] parameter show some disagreement, suggesting that further theoretical investigations are required. Overall, good agreement occurs despite the fact that the current experiments photodissociate molecules at 295 K, whereas previous measurements were conducted at rotational temperatures of about 15 K.