THE photolysis of nitromethane has been carried out with both continuous (see ref. 1) and flash sources (see ref. 2), but for continuous photolysis no concurrent spectroscopy has been reported. This is in general true of most photolysis experiments, since the steady-state concentration of intermediates produced by continuous sources is usually insufficient to record spectra without depending on some matrix isolation procedure. We report here on gas phase continuous wave photolysis experiments using opto-acoustic detection of the energy absorbed by intermediates from a continuous wave (CW) tunable dye laser3. This allowed the detection of intermediate concentrations of ~ 1013 mol cm-3. Opto-acoustic detection was discovered by Bell4 nearly a century ago, but was recently revived as a means of monitoring low concentrations of pollutant molecules5,6. In our experiments the intermediates act as the "pollutant" molecules to be detected. © 1976 Nature Publishing Group.