(a) Schematic of the experimental setup (L, lens; IF, interference filter; Pol., Brewster polarizer; PMT, photomultiplier tube; PEM, photoelastic modulator; QWP, and quarter-wave plate). The axis is defined by the direction of the left circularly polarized (LCP) light pulse that dissociates HBr or HCl to produce SPH. The counterpropagating probe laser excites the SPH from the to the state, and the resulting fluorescence is first polarized and collected by a photomultiplier tube, collecting fluorescence perpendicular to , but linearly polarized parallel to . The complete rotation of the side arm of the vacuum chamber, including the polarizer and the detector, can be achieved so as to collect fluorescence that is polarized parallel to .
Scan of the probe laser over the transition of H atoms produced from a nude ion gauge and with a translational temperature of about . No fine-structure resolution is visible. Measurement of the polarization ratio at three different probe wavelengths within the profile (solid squares) shows that the ratio varies strongly with probe wavelength, demonstrating some degree of fine-structure resolution (see text). In contrast, measurements from H-atom photofragments from the photodissociation of HCl at about , with speeds of about (open circles), show that the ratio does not vary significantly over a larger wavelength range and thus does not show any fine-structure resolution.
Measurement of the polarization ratio as a function of HCl pressure. At higher pressures, the ratio decreases from the ideal value of 2.5, indicating depolarization from collisions.
(a) Energy-level detection scheme, showing that no signal is detected in the limit of no spin-orbit coupling. (b) Similar to (a), but with spin-orbit coupling, showing that only the spin state is detected using LCP probe light.
The dependence of the sensitivity reduction factor on the polarization ratio , from Eq. (6), showing that the reduction in sensitivity in the detection of the spin polarization of H atoms is small provided the ratio is close to a maximal value of 2.5 (see Fig. 3).
Adiabatic electronic potential energy curves of HCl, showing the five states that are relevant to the present study: , , , , and . The potential energy curves for HBr are similar (see Fig. 1 of Ref. 43).
SPH detection from HCl photodissociation at . (a) Experimental fluorescence signals and . (b) The spherical velocity distribution of the SPH, showing the SPH polarization as a function of angle with respect to the laser propagation axis, showing , the polarization component parallel to the atom recoil direction , and , the polarization component perpendicular to . Both and have positive projections along . The one-dimensional projection of this distribution gives the experimental signals. (c) The sum and difference of traces in (a). The sum trace depends only on the velocity distribution of the H atoms, and the difference trace is proportional to the SPH polarization [see Eqs. (7a) and (7b)].
Analysis of the experimental profiles, using Eq. (7), yields values of and for the H-atom photofragments from the photodissociation of HCl, shown here along with the results from the photodissociation of HBr (solid squares), from Ref. 41. These values are compared with ab initio calculated values (crosses), from Refs. 43 and 44, and values inferred from the Br and Cl cofragment polarizations (open circles), from Refs. 10 and 13. The lower error bars represent of the fitted values, determined by a Monte Carlo sampling procedure (Ref. 49). The upper error bars also include the uncertainty in the degree of circular polarization of the light.
Spin-polarized H atoms from the photodissociation of HCl at a pump-probe delay of . Only the H atoms with velocities that are nearly parallel or antiparallel to the probe laser propagation direction remain in the probe volume and are detected.
Scheme for the spin-state specific ionization of hydrogen atoms. Linearly polarized excitation of the transition is followed by two-photon circularly polarized excitation of the transition, followed by ionization. Resolution of the fine structure is not required.
Electronic states involved in the photodissociation of HCl and HBr. represents the halogen atom (Cl or Br). The product branching into ground and excited halogen atoms has been measured to be 0.59 (Cl) and 0.41 for HCl and 0.86 (Br) and 0.14 for HBr. The corresponding spatial distributions of halogen atoms are described by (Cl), , (Br), and . The orientation for ground-state atoms was reported to be for Cl and for Br ( error bars). In combination, these measurements are sufficient to determine the experimental branching into the five product electronic states, as given in the last two columns of the table for HCl and HBr.
A summary of previously published experimental and ab initio vales of polarization parameters and for the halogen-atom photofragments from the photodissociation of HCl and HBr (Refs. 10, 13, 43, and 44). Note that the sign of the parameter used here has been corrected from Refs. 10 and 13. The uncertainties represent of the values.
Analysis of the experimental profiles, using Eq. (7), yields values of and for the H-atom photofragments from the photodissociation of HCl, shown here along with the results from the photodissociation of HBr, from Ref. 41. These values are compared with ab initio calculated values, from Refs. 43 and 44, and values inferred from the and cofragment polarizations, from Refs. 10 and 13. The uncertainties represent of the values. For the present work, the upper error bars also include uncertainty in the degree of circular polarization of the light (see text for details).
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