A novel feature of intramolecular vibrational redistribution in propargyl alcohol and propargyl amine.

Intramolecular vibrational redistribution (IVR) from the terminal acetylene mode nu(HC) has been studied for four molecules: H-C[Triple Bond]C-CH(3) (propyne), H-C[Triple Bond]C-CH(2)Cl (propargyl chloride), H-C[Triple Bond]C-CH(2)OH (propargyl alcohol), and H-C[Triple Bond]C-CH(2)NH(2) (propargyl amine). The experiments were performed with the room-temperature gases. The transition mid R:0-->mid R:1 in the mode nu(HC) was pumped by a short laser pulse. Anti-Stokes spontaneous Raman scattering was used as a probe. The measured parameters were the de-excitation rate W and the dilution factor sigma defined as the relative level of the residual energy in the nu(HC) mode at long pump-probe delay times. The pair of these values {W,sigma} allowed us to determine the density rho(eff) of those vibrational-rotational states, which are involved in IVR from state mid R:1. For two molecules, HCCCH(3) and HCCCH(2)Cl, the experimental results were consistent with the suggestion that all close vibrational-rotational states with the same total angular momentum J and symmetry participate in the IVR regardless of the other rotator quantum number K (in the case of HCCCH(3)) or K(a) (in the case of HCCCH(2)Cl) and the vibrational quantum numbers as well. For the other two molecules, HCCCH(2)OH and HCCCH(2)NH(2), this effect was also present, yet the experimental results revealed certain restrictions. We have obtained a satisfactory theoretical fit with the assumption that the low-frequency torsion vibration of the hydrogen atom in the hydroxyl group (in the case of HCCCH(2)OH) or hydrogen atoms in the amine group (in the case of HCCCH(2)NH(2)) does not participate in the IVR. This assumption can be treated as a challenge to future studies of these molecules by high-resolution spectroscopy and various double-resonance and pump-probe techniques.

Cavity ringdown spectroscopic detection of nitric oxide with a continuous-wave quantum-cascade laser.

A spectroscopic gas sensor for nitric oxide (NO) detection based on a cavity ringdown technique was designed and evaluated. A cw quantum-cascade distributed-feedback laser operating at 5.2 mum was used as a tunable single-frequency light source. Both laser-frequency tuning and abrupt interruptions of the laser radiation were performed through manipulation of the laser current. A single ringdown event sensitivity to absorption of 2.2 x 10(-8) cm(-1) was achieved. Measurements of parts per billion (ppb) NO concentrations in N(2) with a 0.7-ppb standard error for a data collection time of 8 s have been performed. Future improvements are discussed that would allow quantification of NO in human breath.