Pulsed molecular-beam, diode-laser spectrometry using rapid scanning techniques

Publisher Website
Google Scholar
Abstract
We describe a diode‐laser spectrometer for obtaining direct absorption, rovibrational spectra of monomers and/or weakly bound, molecular complexes which are found in supersonic expansions. The spectrometer incorporates a tunable, semiconductor diode‐laser source and a pulsed‐gas slit nozzle. White cell optics are used in the vacuum chamber to increase effective path length, and a Fabry–Perot etalon is used for relative frequency calibration. Stabilization of the source output is accomplished by locking onto a zero crossing of the etalon fringe‐spacing pattern with a gated integrator. The diode laser is scanned rapidly (∼0.2 cm−1/ms) to modulate absorption signals at frequencies which can be electronically filtered from source noise. For 2000 scans, absorbances as small as 1.3×10−5 (0.003% absorption) can be detected. Amplitude fluctuations in the detected signal due to interference effects in the optics and gain variations in the diode laser are eliminated by recording data with and without gas flow from the nozzle, then performing the appropriate subtractions. Because source drift and multiple crossing‐angle effects contribute ≤0.0005 cm−1, observed linewidths (0.003 cm−1) were determined to be laser limited. Data obtained on the van der Waals molecule (Ar⋅Co) are presented and discussed.