Temperature dependent time resolved mid-IR photoacoustic spectroscopy of a nerve gas simulant DMMP

The paper reports the temperature dependent pulsed photoacoustic spectroscopy of Dimethyl methylphosphonate (DMMP) a nerve gas simulant between 50 and 180 °C temperature range. The time domain PA spectra are recorded using two mid-IR wavelengths i.e. 3374 nm, 3495 nm of pulse duration 1.5 ns at 1 kHz repetition rate obtained from optical parametric oscillator. Two anti-symmetric stretching vibrational modes of (CH₃P) and (CH₃O) groups of DMMP molecules have very strong vibrational peaks at 2861.2 cm^-1 (3495 nm) and 2963.8 cm^-1 (3374 nm), respectively. In addition, we have also recorded the PA spectra of acetone at the vibrational frequency 3115.2 cm^-1 (3210 nm), which is the strong vibrational mode of CH band. The comparison of two PA spectra of DMMP and acetone recorded using similar PA cavity help us to understand the effect of other functional groups with respect to different excitation wavelengths. The presence of additional acoustic modes in the PA spectra of DMMP (3374 nm) above the boiling point confirms the slow process of thermal decomposition. Finally, the low level detection limit of DMMP in air is of the of the order of 0.91 ppbV.