Pressure broadening tuning

With techniques of sub-Doppler spectroscopy, even small collisional broadening effects can be investigated with high accuracy. One example is the measurement of pressure broadening and shifts of narrow Lamb dips (Sect. 2.2) of atomic and molecular transitions, which is possible with an accuracy of a few kilohertz if stable lasers are used. The most accurate measurements have been performed with stabilized HeNe lasers on the transitions at 633 nm [975] and 3.39 um [976]. When the laser frequency co is tuned across the absorption profiles of the absorbing sample inside the laser resonator, the output power of the laser Pl(co) exhibits sharp... 

The mechanical or electro-optical modulation of CW lasers applied in these initial experiments was soon superceded by the use of injection-locked pulse-amplified pump lasers in combination with quasi-CW probe lasers [45,80]. When the probe laser has a higher frequency than the pump laser, one obtains a Raman loss or inverse Raman spectrum when the probe laser has the lower frequency, a stimulated Raman gain spectrum is observed by tuning one of the two lasers. Through the pulse amplification of the pump laser, its linewidth is increased over that of a CW laser to between 60 and 100 MHz (0.002 and 0.003 cm ). However, this was at first fully satisfactory, because the resolution in these experiments is largely determined by Doppler and residual pressure broadening. Moreover, the development of injection seeded Nd YAG lasers with smooth pulses has lowered the pump laser linewidth [81] to about 30 MHz. The construction of a seeded Nd YAG laser with longer pulses (35-45 ns) further reduced the linewidth to 10 MHz [82]. 

A number of infrared gas lasers can oscillate on many closely spaced rotational lines of a vibrational transition. Examples are CO2, N2O, HF, DF, or H O lasers. If the pressure in the gas discharge can be made sufficiently large to increase the pressure-broadened linewidth beyond the spacings between adjacent lines, a quasicontinuous gain profile is formed which allows continuous tuning over a larger spectral interval [7.17]. 

Fig. 15. Pressure tuning hole burning spectroscopy of protoporphyrin IX in a glass, (a) Absorption spectrum. The arrow indicates the frequency where the pressure shift vanishes. The corresponding hole is shown in the inset, (b) Shifts and broadening AT of the hole with pressure as a function of burn frequency [From J. Gafert, J. Friedrich, and F. Parak, J. Chem. Phys. 99, 2478 (1993)].

Recently, very sensitive and selective measurements became possible by tunable diode laser absorption spectroscopy (TDLAS). Diode lasers that lase in the mid-infrared region give extremely high resolution (3 x 10 " cm ) and can tune the emission line to one of many vibration-rotation bands by changing the laser temperature and current (10-100K, 0.1-2.0 A). The TDLAS measurement is usually carried out at reduced pressure to avoid band broadening due to molecular collision. Practically interference-free measurements are possible with a typical detection limit of sub-ppbv levels (100 m path, at 25 Torr), although the TDLAS system is still expensive and under development. 

Most measurements up to now have been performed with He-Ne lasers on visible transitions [12.1] or on the 3.39 ym line [12.2] with absorption cells inside the laser resonator. In the infrared region CO2 lasers have also been used [12.3]. The laser output lL(tJ >) is monitored while the laser frequency w is tuned across the Doppler-broadened absorption line and the Lamb dip at the center of the absorption profile occurs as a peak in the laser output. The line profile and the center frequency wq of this inverse Lamb dip are measured as a function of pressure in the absorption cell. Figure 12.2 shows the half-... 

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