Sub-Doppler laser absorption spectroscopy of the AΠi − XΣ (1, 0) band of CN: Measurement of the N hyperfine parameters in AΠ CN

Measurements of multiple rotational lines in the (1, 0) band of the A²Π_i − X²Σ⁺ “red” system of the cyanogen radical (CN) at sub-Doppler resolution are reported. The CN radical was produced by 193 nm photodissociation of NCCN (ethane dinitrile) and detected with a Ti:sapphire ring laser operating near 10 900 cm⁻¹. The sample was exposed to a weak, frequency-modulated probe beam and a strong, counterpropagating bleach laser beam. Demodulated probe laser signals display absorption and dispersion features derived from Doppler-free saturation of the hyperfine components as the laser scans across the central region of a Doppler-broadened rotational line spectrum. Hyperfine-resolved transition frequencies were combined with known ground-state X²Σ hyperfine term values to determine A²Π state hyperfine term values, which were analyzed in terms of an effective Hamiltonian for the A²Π state. All the expected hyperfine and ¹⁴N quadrupolar parameters were determined and their values analyzed in terms of a simple molecular orbital picture of the bonding in the radical. Higher sensitivity obtained with 400 kHz amplitude modulation of the bleach laser and additional phase-sensitive detection allowed hyperfine splittings in some rotational lines of ¹³C¹⁴N to be observed in natural abundance. Excited state hyperfine splittings were determined for a selection of rotational states, but not enough to determine the ¹³C hyperfine parameters.     

Electron-atomic-hydrogen “elastic" scattering in the presence of a laser field

Laser-assisted electron-atomic-hydrogen “elastic" scattering is studied in the first Born approximation. The initial and final states of projectile electron are described by the Volkov wavefunctions; the dressed state of target described by a time-dependent perturbative wavefunction in soft photon approximation. The laser modified cross-sections are calculated in two distinct geometries for laser polarization either parallel or perpendicular to the incident direction of electron. The numerical results shows that the multiphoton cross-sections oscillate by a few orders over the whole scattering angular region. The results for a parallel geometry oscillate more frequently in the intermediate angles; while the results for a perpendicular geometry oscillate more frequently in the forward and backward angles. At large scattering angles, the sum rule of Kroll and Watson is noticeably violated. The laser modification on summed total cross-section increases with field strength, but decreases with field frequency and polarization deviation from the incident direction. Received 18 October 2001 and Received in final form 26 December 2001     

On the theory of the relativistic cross-sections for stimulated bremsstrahlung on an arbitrary electrostatic potential in the strong electromagnetic field

On the base of relativistic generalized eikonal approximation wave function the multiphoton cross-sections of a Dirac particle bremsstrahlung on an arbitrary electrostatic potential and strong laser radiation field are presented. In the limit of the Born approximation the ultimate analytical formulas for arbitrary polarization of electromagnetic wave have been obtained and numerically analyzed. Received 5 April 2001 / Received in final form 18 March 2002 Published online 28 June 2002     

Dark-line in optically-thick vapors: inversion phenomena and line width narrowing

An analysis of the phenomenon of coherent population trapping as observed by means of the dark state in the case of optically thick vapors in alkali metal atoms is presented. Very different behaviors are predicted for the observation of the dark state in the fluorescence and transmission spectra of the same atomic sample when the optical length is not negligible. Among other effects, the dark line observed in the fluorescence signal may look inverted appearing as a bright line, while in the transmission signal a narrowing of the line width resonance line is observed for an increase of the atomic density. In the pure three-level scheme a subnatural width is predicted. These effects, related to the electromagnetically induced transparency phenomenon, are readily observed in experiments, which are found to be in excellent agreement with the theory developed. Received 28 March 2001 and Received in final form 3 October 2001     

Laser influence on positron-antiproton radiative capture collision

The laser-assisted radiative capture between a positron and an antiproton is studied in detail. The theoretical results show that the cross-section for antihydrogen formation is significantly reduced with the application of a laser background. This effect is most marked when the laser polarization is parallelto the incident direction. Received: 13 November 1998 / Received in final form: 16 March 1999     

Su(4) approach to fourfold degenerate electronic states of some hexafluoride molecules

We show that two appropriate realizations of the su(4) algebra allow the construction of all electronic operators needed for the study of vibronic and rovibronic interactions in a G^′ electronic state. In each case a full bosonic realization is made and all matrix elements are calculated. Illustrations of our formalism and comparisons with previous approaches are made in the case of ν₅(F_2g) and ν₃(F_1u) modes.     

Molecular photodynamics involved in multi-photon excitation fluorescence microscopy

We present a simple model for calculating the fluorescence generated by the multi-photon excitation (MPE) of molecules in solution. The model takes into account internal molecular dynamics such as ground-state depletion due to inter-system crossing (ISC), as well as external molecular dynamics associated with diffusion into and out of an excitation volume confined in 3-dimensions. Internal and external molecular dynamics are combined by using a technique of linearization of a modified diffusion equation which takes into account the possibility of concentration depletion due to photobleaching. In addition, we discuss the phenomenon of pulse saturation which effectively limits the molecular excitation rate constant in the case of short pulsed excitation. Our results are specifically applied in the context of fluorescence autocorrelation functions and single-molecule detection. In the latter case, we discuss some consequences of high-order multi-photon photobleaching. Finally, we include three appendices to rigorously define the temporal and spatial profiles of an arbitrary excitation beam, and also to discuss some properties of an exact evaluation of concentration depletion due to photobleaching. Received: 9 March 1998 / Accepted: 20 April 1998