T. M. Goyette

The pressure broadening of the 31,3-22,0 transition of water between 80 and 600 K

Abstract The pressure broadening of the 31,3-22,0 transition of water has been measured over the 80 to 600 K temperature region for O2, N2, and He collision partners. Above 250 K the measurements were made in a conventional equilibrium cell. At lower temperatures a newly developed cell which uses collisional cooling to circumvent the temperature limits ordinarily imposed by vapor pressure was used. For helium broadening and for broadening due to O2 and N2 above 140 K, the results could be fit to the usual exponential temperature dependence with n = 0.49(2), 0.85(3), and 0.74(3), respectively. However, below 140 K the O2 and N2 experimental results are smaller than predicted by this simple exponential relation.

COLLISIONALLY COOLED SPECTROSCOPY: PRESSURE BROADENING BELOW 5 K

New results from pressure broadening measurements between 4.3 and 1.7 K for the J=2←1 and 3←2 transitions of CO broadened by helium are reported. Unlike the results of helium pressure broadening experiments at higher temperature, significant variations in cross section occur with changes in temperature. These measurements are compared with the results of close coupling calculations and show both the effects of resonances in the collision channels and energy defects at the very low collision energies. Room‐temperature measurements of helium pressure broadening have also been made on CO, NO, CH3F, and H2S. These provide a normalization for our previous studies of these species at very low temperatures and a basis for the qualitative comparison of the diverse results we have obtained.

Pressure broadening of the millimeter and submillimeter wave spectra of nitric acid by oxygen and nitrogen

Abstract The pressure broadening coefficients for nitric acid (HNO 3 ) interacting with oxygen (O 2 ) and nitrogen (N 2 ) have been measured for 16 transitions in the millimeter and submillimeter region. These transitions represent a good selection of the strong lines in the rotational spectrum of this species and make possible a systematic comparison with earlier theoretical calculations. For broadening by both oxygen and nitrogen we find larger coeffients than predicted by theory. In addition, the state to state variation is higher than calculated, especially for oxygen broadening for which the calculation shows essentially no state to state variation.

Variable temperature pressure broadening of HNO3 in the millimeter wave spectral region

Abstract The O2 and N2 pressure-broadening parameters of the 180,18-170,17, 227,15-217,14, and 290,29-280,28 transitions in the ground vibrational state of HNO3 have been measured in the temperature range between 100 and 380 K. Above 190 K the measurements were made in an equilibrium cell. Below 190 K, a cell with collisional cooling to circumvent the temperature limits imposed by the vapor pressure of the sample gas was used. The data were fit to the usual exponential temperature dependence with resultant n values ranging from 0.67(5) to 0.84(10) for O2 and from 0.62(3) to 0.74(9) for N2.

Spectral purity and sources of noise in femtosecond-demodulation terahertz sources driven by Ti:sapphire mode-locked lasers

Direct measurements of the spectral purity in terahertz femtosecond-demodulation sources are reported and compared to theory. Because these sources operate at very high harmonics (/spl sim/102-10/sup 4/) of the mode-lock frequency, a high spectral purity source is very dependent on a low-jitter femtosecond laser. Conversely, the spectral content of the terahertz sources provides detailed information about timing jitter and stringent tests of models used to describe the jitter. We find that both the behavior of the central core, and the noise skirts of the power spectrum of our sources, can be quantitatively related to measured ripple and continuum amplitude noise on the Ar/sup +/ pump laser by use of modulation theory.

Rotational and vibrational temperatures in a 77 K collisionally cooled cell

Abstract The rotational and vibrational temperatures of CH3F and SO2 in a collisionally cooled cell have been investigated. This cell, which uses liquid nitrogen as the cryogen and has an injector whose temperature can be varied between ∼200 and 1300 K, is a variant of liquid helium based systems which we have previously described. The work reported in this paper shows that the rotational temperature is equal to that of the buffer gas, which in turn is variable from 77 K up to the trapping temperature of the spectroscopically active gas by adjustment of the wall temperature of the cell. The vibrational temperature is determined by the vibrational temperature with which the gas is injected, the number of collisions with the buffer gas, and the vibrational deactivation probability of these collisions. Thus, it is shown that the vibrational and rotational temperatures of spectroscopic samples can be separately adjusted over a wide range of temperature with a very simple experimental device. Applications are discussed.

Pressure broadening of NO2, CF2Cl2, HDO and HOOH by O2 and N2 in the millimeter wave region

Abstract The pressure-broadening coefficients of four molecules of atmospheric significance (NO2, CF2Cl2, HDO and HOOH) have been measured in the millimeter wave region. For each species, two transitions near 230 GHz were broadened by both N2 and O2, for a total of 16 different pressure-broadening measurements. Intercomparisons among the results of these measurements, as well as our recent more extensive measurements on HNO3, are considered.

The pressure broadening of NO2 in the millimeter and submillimeter wave spectral region

Abstract Pressure-broadening parameters for 19 transitions of the millimeter and submillimeter wave spectrum of NO 2 have been obtained experimentally. Results for broadening by N 2 , O 2 and He are presented and compared with earlier theoretical calculations of the N 2 and O 2 parameters. A discussion of the relation between theoretical calculations and experimental measurements is presented.

PRESSURE BROADENING OF HDO BETWEEN 90 K AND 600 K

The results of recent studies of SO2 pressure broadened by H2, He, 02, and N2will be presented. This is part of the variable temperature pressure broadening studies ofvarious molecules between 90 K and 600 K. The 4(2,2)-3(1,3), 13(1,13)-12(0,12), and26(4,22)-26(3,23) transitions of the ground vibrational state of SO2 are studied between236 and 500 K. Measurements at other temperatures are in progress at this time. Resultsfor the temperature dependence of the pressure broadening parameters will be discussedand compared with the results of previous studies on other molecules. Any trend in the 300K broadening parameters and temperature coefficient as a function of their quantumnumbers will be noted.