Spectral Line Shape Studies at Toronto

A high resolution spectrometer, constructed in the late 1980's at the University of Toronto, is now fully operational. It is used for recording precise Raman and infra-red spectra.

Although the basic spectroscopy of molecules is well understood, precise measurements of pressure broadening, line shifting, line mixing, and line shapes now provide critical tests of theoretical calculations. Until recently, most line shape measurements could not distinguish between different line shape models. Furthermore, effects related to state-to-state transfer rates, finite duration of collisions, and the speed dependence of width and shift cross-sections can now be probed and used to test theories.

Both types of spectroscopy require two CW, frequency stabilized lasers of which one is tunable. In stimulated Raman spectroscopy, an argon ion laser serves as the pump soure and a dye laser (Coherent 699-29) serves as the tunable probe. For infra-red spectoscopy, the two laser beams are combined in a lithium iodate crystal to generate infra-red radiation at the difference frequency. A Fabry-Perot permits the calibration of the frequency axis with a resolution of 1 MHz. A stabilized He-Ne laser provides a fiducial reference for observing and correcting for any long-term variations in the Fabry-Perot. Although the absolute frequency accuracy is limited to .02 /cm 600 MHz by the variation (daily) of the frequency of the ion laser and the accuracy of the wavemeter in the dye laser, we routinely measure pressure shifts with a 10 MHz accuracy relative to the line origin.

Hydrogen molecules are the simplest diatomic molecules for which theoretical calculations are now just tractable with modern day computers. The wavelengths available from Ar+ and dye lasers fortuitously coincide with the requirements for recording Raman spectra of the Q branch in D2 and permit a high signal-to-noise ratio often exceeding 1500 with a 1 s integration time. The stability of CW lasers allows the detection of shot-noise limited spectra. A typical spectrum of the Q(2) line in D2 also exhibits an asymmmetric structure due to line mixing.

In addition to improving our understanding of line shape models, measurements of the line widths and integrated intensities are relevant to such fields as atmospheric pollution studies. In particular, the temperature can be determined from the intensity distribution of rotational spectal lines. Likewise, the pressure can be deduced from the widths of the lines.

Recently completed research includes line shift and width measurements of depolarized Q branch spectra in D2, line shift measurements in C02, non-Lorentzian features in the P and R branches in CO, line mixing in D2, and the widths and shifts of the vibrational O and S branches in D2 at 100K.

Spectral Line Shape Studies Science Group
Recent Publications