Using a powerful, excimer-laser-pumped broadband coherent anti-Stokes Raman scattering (CARS) system, we have observed considerable changes in the populations of room-temperature nitrogen under atmospheric pressure. Pump-induced populations in the first, second, and third vibronic bands of N2 at room temperature resulted in an apparent vibrational temperature of 2550 K. The implications for broadband N2 CARS thermometry are discussed.

Pump-induced population changes in broadband coherent anti-Stokes Raman scattering.

Line shapes and intensities for coherent anti-Stokes Raman scattering (CARS) at saturation laser intensities are determined for nitrogen vibrational Q-branch lines by solving the time-dependent density-matrix equations numerically. We have previously performed measurements of saturated CARS line shapes in pure nitrogen by using nearly Fourier-transform-limited pump and Stokes lasers. The experimental laser pulse shapes, Stark effects, Doppler broadening, and the nonresonant background are incorporated in the numerical calculations. The numerical results show good agreement with the high-resolution measurements of saturated CARS line shapes. The lines show prominent saturation dips, and agreement between theory and experiment is excellent in terms of the depth and the width of the dips. The numerical results indicate that the Doppler effect tends to broaden and to deepen the dip in highly saturated lines, an effect that cannot be explained by a steady-state theory.

Calculation of saturation line shapes and intensities in coherent anti-Stokes Raman scattering spectra of nitrogen

Saturation of coherent anti-Stokes Raman scattering (CARS) spectra of the Q(1) line of the hydrogen (1, 0) vibrational transition was investigated experimentally by using high-resolution lasers and theoretically by solving the time-dependent density matrix equations. The saturation behavior of hydrogen is complicated by the large Doppler width of the resonance and the high rate of velocity-changing collisions relative to dephasing collisions. Experimentally, GARS line shapes and saturation curves were measured in pure hydrogen at pressures of 100 and 3050 Torr. Surprisingly, the measured saturation intensity was found to be less at 3050 Torr than at 100 Torr. The time-dependent density matrix equations were numerically integrated to obtain CARS saturation curves and line shapes. Excellent agreement between calculated and experimental line shapes was obtained at both 100 and 3050 Torr, and the predicted saturation intensity was less at 3050 Torr than at 100 Torr. Based on the good agreement between theory and experiment obtained at 100 and 3050 Torr, the theoretical results were extended over a much wider pressure range, from 0.1 to 100,000 Torr. Below 1 Torr the saturation behavior is independent of pressure because collision times are long compared with times associated with laser excitation of the resonance, and the molecular response is completely transient. Between 1 and a few hundred Torr the saturation intensity increases as the rate of velocity-changing collisions increases. Above a few hundred Torr, however, the saturation intensity begins to decrease because the high rate of velocity-changing collisions ensures that all molecules couple effectively with the Raman-pumping lasers. Calculations show a minimum in saturation intensity at 2000 Torr. For even higher pressures, saturation is controlled by dephasing collisions, and the saturation intensity increases rapidly with pressure.

Saturation effects in coherent anti-Stokes Raman scattering spectroscopy of hydrogen

Chapter 1 – Coherent Anti-Stokes Raman Spectroscopy*

We report collision‐free coherent anti‐Stokes Raman scattering (CARS) spectra of the O2 fragment formed in the photodissociation of ozone at 532 and 578 nm. From these spectra we have extracted the nascent electronic, vibrational, rotational, and translational energy distributions of the photofragments. The O2 fragment is produced only in the ground 3Σ−g electronic state, even though the 1Δg excited state is energetically allowed. High (J≊23 to 51) rotational states in the first five vibrational levels of O2(3Σ−g) are populated in the photodissociation. For both photolysis wavelengths most of the available energy, 65%–67%, appears in translation, while 15%–18% is in rotation, and 17%–18% is in vibration. These energy distributions are discussed in terms of simple models of the dissociation dynamics.

P. Oesterun

Papers

Observation of saturation broadening of the coherent anti-stokes raman spectrum (cars) of acetylene in a pulsed molecular beam