Temperature Dependence Of Line Shift Research Articles

On the temperature dependence of half-widths and line shifts for molecular transitions in the microwave and infrared regions

An expression was developed that correctly models the temperature dependence of the half-width over large temperature ranges and the temperature dependence of the line shift, even for cases where the line shift changes sign. The expression was derived from the expansion of the collisional cross-sections in a series of powers of the relative velocity of the radiator and perturber. Cutting off at two terms yields a complex double power law (DPL) expression, where the real and imaginary parts describe the temperature dependence of the half-width and line shift, respectively. Data were collected for more than 100 thousand transitions and the standard power law expression for the half-width and line shift were compared with the new double power law expression. It is shown that the double power law works well for all transitions, even those that exhibit unusual structure, which the standard power law cannot model. The DPL expression gives better results than the standard power law for all transitions studied and the DPL model gives good results for the temperature dependence of the line shift when it changes sign. The DPL model for the temperature dependence of the line shift was compared with the linear model, which is currently used on the HITRAN database. In all cases the DPL model gave much better results than the linear model no matter what temperature range was considered. The new formalism allows a substantial reduction in the number of parameters that need to be stored in databases and the same expression can be utilized in radiative transfer and simulation codes for both the half-width and line shift.

Excimers in the Lowest Rotational Quantum State in Liquid Helium.

Evidence for helium excimers (He2*) in the lowest allowed rotational quantum state in liquid helium is presented. He2* was generated by a corona discharge in the gas and normal liquid phases. Fluorescence spectra recorded in the visible region between 3.8 and 5.0 K and 0.2 and 5.6 bar showed the rotationally resolved d3Σu+ → b3Πg transition of He2*. Analysis of the pressure and temperature dependence of lineshifts and line intensities showed features of solvated He2* superimposed on its gas-phase spectrum and, in the liquid phase only, pressure-induced rotational cooling. These findings suggest that He2* can be used to investigate bulk helium in different phases at the nanoscale.

Temperature-Dependent Line Shift and Broadening of CO Infrared Transitions

The temperature dependence of lineshift and broadening of the rovibrational transitions R(18) and R(20) of the CO fundamental band, perturbed by Ar, N2, O2, and H2, have been measured with high frequency accuracy and at temperatures between 160 and 270 K in steps of 20 K. A wavelength stabilized tunable diode laser spectrometer has been combined with a low temperature long path cell of 134 m absorption length and 1 m basis length. For all measurements the CO pressure was below 0.1 mbar to avoid self-shift and self-broadening. In case of line broadening the temperature dependence is quite well reproduced by an exponential relation, b(T) = b(T0)(T/T0)−n. For all foreign gases, the exponent n has been obtained (0.53 ≤ n ≤ 0.71) and a value for air has been calculated from the weighted mean values of N2and O2. Within the error limits the magnitudes of all shifts decrease with increasing temperatures, but there is no exponential behavior of the shift versus temperature. The line broadening and shift for CO with Ar and the broadening of CO by N2and O2have been compared to calculations from the semi-classical theory of Robert and Bonamy. Sufficient agreement has been achieved for the line broadening, while the calculated shifts are for all temperatures larger than the measured values.

In Situ High-Resolution FT-IR Study of the Orientational Phase Transition in C60 Film

We have studied the orientational phase transition of a water-free C{sub 60} film by examining the temperature dependence of line shift, half-width, and integrated absorbance of the four fundamental IR-active modes in the C{sub 60} film with high-resolution (0.25 cm{sup -1}) Fourier transform infrared (FT-IR) spectroscopy. A discontinuous line shift of the IR-active mode at 576 cm{sup -1} is observed at about 257 K for the first time. This temperature is consistent with the previous results that the orientational phase transition of C{sub 60} bulk structure occurs at 250-262 K. In addition the temperature dependence of the line shift for the IR-active mode at 526 cm{sup -1} is the opposite of those for the other modes. The reason for this difference can be explained qualitatively on the basis of the relation between the intermolecular interaction and the motion of atoms for each IR mode. 27 refs., 7 figs.

The temperature dependence of lineshifts, linewidths and line intensities of methane at low temperatures

Employing the techniques that we have described previously for measuring infrared (i.r.) lineshifts, linewidths and line intensities with a tunable diode laser, we have measured these parameters for lines in the R(4) and R(7) multiplets of the v 4-fundamental band of 12CH 4 at 161, 209 (or 218) and 295 K using He, Ne, Ar, H 2, N 2, O 2, and air as the perturbers. The temperature dependence of the collision-induced lineshift is markedly different from that of the corresponding collision-broadened linewidth.