David S. Perry

Multipass cell for molecular beam absorption spectroscopy

We report a two-spherical mirror multipass cell which allows efficient coupling of a laser beam to a collimated molecular beam at the center of the cell. With near-concentric alignment and the laser beam coupled into and out of the cell at the edges of opposite mirrors, an approximately parabolic pattern of reflections is observed on each mirror. Aligned for forty to fifty passes, the cell is used for direct absorption spectroscopy of jet-cooled acetylene, propyne, and 1-butyne in the 3-microm region. Sub-Doppler linewidths as narrow as 12 MHz have been recorded.

Vibrational state mixing of individual rotational levels in 1‐butyne near 3333 cm−1

The high resolution infrared spectrum of jet‐cooled 1‐butyne is reported for the acetylenic C–H stretch. The absorbance of an F‐center laser beam is enhanced by a multiple reflection cell surrounding the free jet. Although the spectrum in the gas phase (300 K) is a featureless rotational contour, at 3 K sharp features are observed. For J’=0, 1, 2, the K’a =1 lines are spectroscopically well‐behaved, but the Ka =0 lines are split into multiplets. Analysis of the K’a =1 lines gives the constants (B’+C’)/2=0.143u200982(11) cm−1, (B’−C’)/2=0.007u200920(11) cm−1, and ν0+A’−A‘=3333.0182(9) cm−1. The quantities in parentheses are two standard deviations in units of the last digit. It is concluded that the Ka =0 levels are mixed with the bath of background vibrations by anharmonic matrix elements the order of 0.01 cm−1.

Sliced-jet spectroscopy: a new technique for sub-Doppler spectroscopy of jet-cooled molecules

Abstract Direct absorption spectra of jet-cooled molecules are recorded by multipassing an infrared laser beam through a nozzle expansion and with all light rays perpendicular to the nozzle axis. A small blade is placed just upstream of the laser crossing to obstruct the passage of molecules that would otherwise absorb at the center of the Doppler profile. For the acetylene R1 line at 3300 cm -1 , the resulting holes are as narrow as 12 MHz. The 1-butyne 3333 cm -1 band is used to illustrate the technique.