Guohe Sha

Cavity ring-down spectroscopy in the liquid phase

Author Institution: UNIVERSITY OF AKRON; DEPARTMENT OF CHEMISTRY, STATE KEY LABORATORY OF MOLECULAR REACTION DYNAMICS; DALIAN INSTITUTE OF CHEMICAL PHYSICS, STATE KEY LABORATORY OF MOLECULAR REACTION DYNAMICS

Evidence for quantum interference in collision‐induced intramolecular energy transfer within CO singlet–triplet mixed states

The quantum interference effect associated with one type of radiationless transition, the collisional energy transfer between singlet–triplet mixed molecular states, is studied. The experiments are conducted on CO(A 1Π,e 3Σ−)–He(Ar) via the ultrasensitive optical–optical double resonance multiphoton ionization (OODR‐MPI) technique which measures state‐to‐state cross sections to an accuracy of ±10%, irrespective of the lifetime of the excited state. Distinct evidence of a quantum interference effect on the transfer rate has been obtained for mixed state CO intramolecular energy transfer processes. A simple, explicit expression for the cross section for mixed state energy transfer, based on the first order Born approximation of time dependent perturbation theory, is derived. The use of a transition phase angle θST is incorporated in the expression to describe the phase angle difference between singlet and triplet channels. This greatly refines the existing theory of quantum interference for collisional proc...

FURTHER STUDY ON COLLISIONAL QUANTUM INTERFERENCE EFFECT IN ENERGY TRANSFER WITHIN CO SINGLET-TRIPLET MIXED STATES

In our previous study [G‐H Sha, J‐B. He, B. Jiang, and C‐H. Zhang, J. Chem. Phys. 102, 2772 (1995)], a concept of interference phase angle (θST) has been formally introduced to define the coherence effect between singlet and triplet energy transfer channels for mixed states. In this contribution, we have measured θST for various monoatomic (He, Ne, Ar) and diatomic (H2, N2) collision partners at temperatures of 77 K and 470 K. Via a new data processing approach, θST is fitted more accurately than earlier approximations. Our experimental results show that θST increases with the polarizability of monoatomic collision partners, while for diatomic collision partners θST is significantly higher than that for monoatomic ones. θST at 470 K for He and Ne is higher than that of 77 K, while an adverse temperature effect on θST has been found for N2. The influence of intermolecular potential and possible complex formation between excited CO and N2 or H2 has been discussed.

Two-color resonant multiphoton ionization study of CO. The rotational energy transfer behavior of CO A1 Πstate

Two-color resonant multiphoton ionization provides a novel technique, complementary to LIF, for studying state-to-state molecular relaxation dynamics. A set of formulae applicable to two-color pulsed laser ionization experiments has been derived to evaluate the rotational energy transfer probability for CO(A)-CO(X)collision pairs. The magnitude of this probability is on the order of 1/10 for rotational quantum jump ΔJ = ±1 and diminishes with increasing ΔJ. For the same > J(<±3), a transition with e/f symmetry conservation is always more probable than that with an e/f change.

Quantitative measurements of O2 b ← X(2,1,0←0) bands by using cavity ring-down spectroscopy

Abstract Quantitative measurements of individual rotational line intensities of molecular oxygen are made for the (0←0), (1←0), (2←0) bands of the b 1 Σ g + ← X 3 Σ g − transition using highly sensitive cavity ring-down spectroscopy (CRDS). The absorption profiles of six rotational lines of the three bands of O2 are measured under pressures up to 1 atm. Corrections have been made to eliminate the effect of laser bandwidth on the absorption intensity measurements. The gas detection limits in our measurement for the three bands are 0.04, 0.80 and 7.00 Torr, respectively. The line strengths in the three bands of O2 have been determined by CRDS and are in good agreement with previous results using a multiple pass absorption cell.

Angular momentum reorientation in CO(A 1Π)–He rotational energy transfer studied by optical–optical double resonance multiphoton ionization spectroscopy

An optical–optical double resonance multiphoton ionization (OODR‐MPI) technique has been developed for measuring the angular momentum reorientation in CO(A 1Π)–He inelastic thermal collisions. In this scheme, two‐photon pumping of CO(A 1Π) by using a circularly polarized laser creates a highly anisotropic oriented angular momentum distribution in CO(A 1Π). A second counterpropagating circularly polarized laser probes the oriented CO(A 1Π) via 1+1 photon resonance ionization. A depolarization factor (D), which can be calculated from the measured intensity ratios between R and P branches in the OODR‐MPI spectrum, is introduced to characterize the amount of collisional reorientation in the J→J’ rotational energy transfer (RET). This method has the advantage of both high sensitivity and simplicity in data processing. The experimental results show the propensities that the depolarization factor D increases with ΔJ but decreases with increasing initial J. The D’s are asymmetric with respect to ±ΔJ. For comparis...

Collisional energy transfer studies of electronically excited CO: energy transfer between the three sublevels of CO(e 3Σ−, ν = 1) state

Abstract Study of He atom collision-induced rotational energy transfer between the three F components of CO(e 3 Σ − , ν = 1) state by OODR-MPI spectroscopy shows that the propensity rule for cross sections applicable to all F i s ( i = 1, 2, 3) may be summarized as σ(F i → F i ) > σ(F i → F i + 1 )> σ(F i → F i + 2 ) for N >/ 2. As the quantum number N varies from 3 to 12, this propensity becomes stronger in that σ(F i → F i ) remains almost unchanged, while σ(F i → F i + 1 ) and σ(F i → F i + 2 ) diminish very rapidly. The above rules are in line with the theory of Corey, Alexander and Schaefer (J. Chem. Phys. 85 (1986) 2726). Absolute cross sections have also been obtained which may serve to calculate the tensor opacities in the future.

Long-lifetime and asymmetric singlet oxygen photoluminescence from aqueous fullerene suspensions.

The photoexcited aqueous fullerene (C60) suspension was shown to exhibit an asymmetric photoluminescence (PL) spectrum, which, different from the symmetric spectrum observed previously in C60 solutions or suspensions, still stems from the characteristic phosphorescence of singlet oxygen (O2(a(1)Δ)) owing to its dependence on oxygen concentration. In contrast to the microsecond-level lifetime of O2(a(1)Δ) in water solutions, that in our C60 suspensions was measured at room temperature to be relatively long, about 2-3 ms, which is ~1000 times longer than the value reported by Bilski et al. The physical mechanism for the asymmetric O2(a(1)Δ) PL from C60 suspensions was studied in depth, indicating that it in fact originates from O2 molecules trapped in the C60 lattice within the suspended C60 aggregates (nC60). This mechanism, which can explain well our above results, was further validated by the nC60s high-resolution transmission electron microscopy (HRTEM) images with lattice fringes and the experimental temperature dependence of O2(a(1)Δ) lifetimes in nC60 suspensions. Our findings suggest that the bulk-phase O2(a(1)Δ) in aqueous nC60 suspensions results from the diffusion of the O2(a(1)Δ) generated within the interior of nC60 aggregates.

Collisional quantum interference effect on rotational energy transfer in an atom-diatom system

Abstract The theoretical model of collisional quantum interference (CQI) in intramolecular rotational energy transfer is described in an atom–diatom system, based on the first Born approximation of time-dependent perturbation theory and considering a long-range interaction potential. The relation between differential and integral interference angles is obtained. For the CO A 1 Π(v=0)/ e 3 Σ − (v=1) –He collision system, the calculated integral interference angles are consistent with the experimental values. The physical significance of interference angle and the essential factors it depends on as well as the influence of the short-range interaction on CQI are discussed.

Two‐color study of Autler–Townes doublet splitting and ac Stark shift in multiphoton ionization spectra of CO

By using a two‐color scheme, we have given a closer scrutiny to the Autler–Townes doublet splitting and ac Stark shift in the 2+2‐photon ionization spectra of CO Ion A 1(v’=4)←X 1Σ+(v‘=0) transition in the presence of another strong laser field, which is resonant with the transition C 1Σ+(v’=0)←A 1Π(v’=4)P10 branch. All the observations of ac Stark shift/broadening as a function of the perturbing light frequency and its spatial homogeneity strongly support our previous theoretical interpretation for one‐color studies. A qualitative interpretation based on the dressed state theory, which is being quantified, for the Autler–Townes splitting as well as the ac Stark effect has been given. In particular, the magnitude of the Autler–Townes splitting is found to vary with the square root of the perturbing light intensity, in line with the dressed state theory.