Jinchun Xie

Selection rules for the photoionization of diatomic molecules

In the photoionization of the diatomic molecule AB to yield AB++e− the photoelectron may be charatcterized by a partial wave expansion in terms of its orbital angular momentum quantum number l. For a given value of l, conservation of angular momentum implies that transitions can only occur for ΔJ=l+ (3)/(2) , l+ (1)/(2) , ... ,−l− (1)/(2) , −l− (3)/(2) , where ΔJ=J+−J is the change (half‐integer) in the total angular momentum (excluding nuclear spin) of the AB+ ion rovibronic level and the AB neutral rovibronic level. Other selection rules are ΔΩ=−λ+ (3)/(2) , −λ+ (1)/(2) , ... , −λ− (3)/(2) , and ΔM=−ml+ (3)/(2) , −ml+ (1)/(2) , ... , −ml− (3)/(2) . In addition, for Hund’s case (a) and case (b) coupling, ΔS=S+−S=± (1)/(2) , ΔΣ=± (1)/(2) , and ΔΛ=−λ, −λ±1. Parity selection rules have been derived for transitions connecting levels described by one of the four coupling schemes, Hund’s case (a), case (b), case (c), and case (d). In particular, for a case (a)–case (a) transition, ΔJ−ΔS+Δp+Δs+l=odd, where the ...

Rotationally state-selected HBR+: Preparation and characterization

Abstract Rotationally state-selected HBr + ions have been prepared by 2 + 1 resonance-enhanced multiphoton ionization (REMPI) via Rydberg states of HBr, and characterized by laser-induced fluorescence (LIF) using the HBr + A 2 Σ + -X 2 Π system. The experiment has been conducted under bulb conditions of 10–50 m Torr. The rotationally unrelaxed distribution of HBr + internal states shows that the ionization step can be highly selective, depending on the intermediate Rydberg state chosen.

Laser diode cavity ring-down spectroscopy using acousto-optic modulator stabilization

By using an acousto-optic modulator, we have stabilized a free-running continuous wave (CW) laser diode in the presence of strong reflections from a high finesse Fabry–Perot resonator. The laser diode linewidth can be stabilized from several MHz, for high resolution spectroscopy of species at low pressures, to several hundred MHz, for lower resolution spectroscopy of species at atmospheric pressures. We demonstrated CW cavity ring-down spectroscopy of water vapor at both 1 atm and 5 Torr. We achieved ring-down repetition rates of 10–50 kHz, and a noise level of 2×10−8 cm−1.

Rotational line strengths for the photoionization of diatomic molecules

We derive an expression for the probability that a diatomic molecule AB(n,v,N) in the electronic state n, vibrational state v, and rotational level N yields upon photoionization AB+(n+,v+,N+), where we assume Hund’s case (b) coupling. Our result is formally equivalent to the previous work of Buckingham, Orr, and Sichel [Phil. Trans. Roy. Soc. London, Ser. A 268, 147 (1970)] but differs substantially in that we use spherical tensor methods, which provide insight into the photoionization dynamics in terms of the contribution of different multipole moments. The total interaction term is given by the tensor product of the electric dipole moment operator T(1,μ0) and the multipole moment tensor T(l,m) describing the photoelectron in the lth partial wave. The interaction term is further simplified into a sum of reduced multipole moments T(k,p), where k=l±1 and p=μ0+m. For an isotropic distribution of initial states, the transition probability is given by P(N,N+)=1/3∑k Sk(N,N+)‖μ(k,q)‖2, where the factor of 1/3 ...

Near-infrared cavity ringdown spectroscopy of water vapor in an atmospheric flame

We have used cavity ringdown spectroscopy CRDS to measure near-infrared overtone transitions of water in . atmospheric flames propane premixed jet and laminar methane-air flat flame burner . The strong signal output with a well-defined laser beam direction and the insensitivity to strong background emission present in hostile environments make CRDS ideal for the study of combustion environments. We have obtained spectra of water vapor from within a flame and extracted a profile of the rotational temperature and concentration of water vapor as a function of distance from the plane burner surface. q 1998 Elsevier Science B.V.

Determination of absolute thermal rate constants for the charge-transfer reaction DBr+(2∏I,v+)+HBr→HBr+(2∏I′,v′+)+DBr

The charge transfer reaction DBr+(2∏I,v+,J+)+HBr→HBr+(2∏I′,v′+,J′+)+DBr is studied in a state-to-state manner under thermal conditions in a slowly flowing gas mixture of HBr and DBr. The DBr+ reagent is prepared in a selected vibronic level by using (2+1) resonance-enhanced multiphoton ionization. The HBr+ product is detected in a quantum-state-specific manner using laser-induced fluorescence. From the measurements of the molecular density and the populations of both HBr+ product and DBr+ reagent, the absolute thermal rate constants k(i,v+→i′,v′+) are determined for this charge-transfer process. The rate constants for near-resonant charge transfer in which Δv+=0 and Δi=0 are much large than charge-transfer channels in which either Δi≠0; the smallest rate constants are for those channels in which both Δi≠0 and Δv+≠0. The rotational distribution of the HBr+(i′,v′+) products fits a temperature well in each case. For near-resonant charge transfer, the rotational temperature is slightly warmer than thermal, wh...

Rate constants and products for the reaction of HBr+ with HBr and DBr

Abstract The proton transfer channel for the reaction of HBr + with HBr is investigated in a state-specific manner by preparing the reagent ion in a selected internal state by resonance-enhanced ionization (REMPI) and monitoring the disappearance of this ion by laser-induced fluorescence (LIF). A rate constant for proton transfer of (6.7 ± 1.6) × 10 −10 cm 3 /s is found. The reaction of HBr + with HBr and DBr was also studied in a selected ion flow tube. The reactions were found to be quite complex producing charge transfer, proton transfer, and hydrogen transfer products. The rate constants are (5.3 ± 1.3) × 10 −10 cm 3 /s for the removal of HBr + by HBr and DBr neutrals.