Advances of sensitive techniques in laser spectroscopy

Different techniques are used for the sensitive detection of overtone transitions of small molecules, such as H2S, C2H2, OCS, O3 in the visible and near infrared region. The techniques include: phase-modulated or wavelength-modulation absorption spectroscopy with a widely tunable single mode diode laser, opto-acoustic spectroscopy in resonance cells with a color center laser or a Ti:Sapphire laser, and sub-Doppler opto-thermal spectroscopy in cold collimated molecular beams, using a low temperature bolometer as detector. The three techniques are compared with regard to sensitivity and spectral resolution. The mass selective sensitive detection of small metal clusters Li3 and Na3 and their high resolution sub-Doppler spectroscopy, using resonant two-photon ionization with cw lasers, is briefly discussed.