Low-Temperature Rotational Relaxation of N₂ Studied with Resonance-Enhanced Multiphoton Ionization

Abstract

The rotational relaxation of molecular nitrogen has been investigated down to temperatures of about 5 K with a combination of resonance-enhanced multiphoton ionization and supersonic beam time-of-flight techniques. The average rotational relaxation cross section obtained shows a maximum value of 50−60 Å2 at 20−30 K. For lower temperatures this cross section decreases and reaches a value smaller than 30 Å2 at T ≈ 5 K. For temperatures above 30 K, the cross section decreases slowly as the temperature grows and converges approximately to the determinations from other non-jet techniques and theoretical estimates available for T > 80 K. The results are compared to previous measurements from other groups using different methods, and general good agreement is found. However, we observe a significant discrepancy with some of the data from electron-beam-induced fluorescence that yield much larger cross sections for temperatures lower than 30 K.

Lineshape analysis of stimulated Raman spectra of the near‐nozzle region of free jet expansions

“…In this work, we assume equilibrium between both temperatures, because the jets studied are produced at high stagnation pressures that maintain this equilibrium until a large distance from the nozzle. [9,29] The off-axis rotational temperature has been proved [30] to follow Eqn (3) along a perpendicular length of several D, before reaching the lateral shock wave that causes an increase in T. The lateral shock wave produces also an increase of the density, but at a larger distance from the axis than that at which the temperature rises, the same as in normal shock waves. [31] The influence of the lateral shock wave is stronger in regions close to the nozzle and can limit the lowest temperature reached by the flow.…”

“…Among them, highresolution spectroscopic techniques, such as visible absorption, [3] diode-laser infrared absorption, [4,5] Fourier transform infrared (FTIR) absorption spectroscopy, [6,7] Fourier transform microwave (FTMW) measurements, [8] resonance enhanced multiphoton ionization (REMPI), [9,10] cavity ring down spectroscopy, [11] or nonlinear Raman spectroscopies (stimulated Raman Spectroscopy (SRS), CARS, etc), [12,13] have been widely used to measure vibrationrotation spectra of molecules cooled in the expansion and for the analysis of phenomena naturally occurring, or externally induced, in free jets and supersonic beams, like nonequilibrium between the molecular degrees of freedom, [9,10] nucleation and clustering of the gas, [11,12] photodissociation and photon-induced reactions, [14,15] or interactions of molecules with fields. [16] In the spectra obtained from jets with techniques that have a great field depth, like infrared absorption, the measured intensity and the profile of the spectral lines are the result of an integration along the line of sight, containing information about the density, temperature, and velocity of the observed particles within the jet.…”

Inelastic collisions in molecular oxygen at low temperature (4 ⩽ T ⩽ 34 K). Close-coupling calculations versus experiment

“…(16) and (17) downstream in the jet are imposed by the availability of enough elastic collisions. 58 The estimated terminal temperatures for the 230 and 100 mbar O 2 jets are T t∞ = 3.9 and T t∞ = 7.4 K, respectively. (16) and (17) becoming no longer valid.…”

“…The limits of validity of Eqs. 57,58 The point z q is estimated here on the basis of the similarity flow theory using the Reynolds number (Re * ) at the source as a similarity parameter, and the terminal translational temperatures of N 2 at the same values of Re * . If the number of remaining collisions from a point z q is less than one, a progressive freezing of T t for z > z q can be expected, Eqs.…”

MEASUREMENTS OF ISOTOPE EFFECTS IN THE PHOTOIONIZATION OF N 2 AND IMPLICATIONS FOR TITAN'S ATMOSPHERE

“…This mixture then underwent supersonic expansion to form a molecular beam. Although the temperature of the beam was not measured directly, experiments done under similar conditions have yielded N 2 translational and rotational temperatures of 50 K (e.g., Aoiz et al 1999;Mori et al 2005), and little vibrational excitation is expected under these conditions; thus the N 2 beam was translationally, rotationally, and vibrationally cold. The beam was then crossed at 90…”