The paper presents the results of an experimental study of the dynamic characteristics of the laser fragmentation/laser-induced fluorescence (LF/LIF) effects in nitrobenzene vapors under the separate initiation of processes of photofragmentation and fluorescence of fragments by nanosecond laser pulses. It is shown that, due to the inertia of the dissociation mechanism of nitrobenzene molecules, the process of the fragments’ formation continues even after letup of excitation. The highest concentration of fragments is reached in a time several times greater than the standard fragmentation pulse duration of 10 ns. A kinetic model is presented that allows one to trace the temporal dynamics of the LF/LIF process of nitrobenzene vapors under separate excitation. A good agreement between the experimental data and the results of calculation indicates the adequacy of application of the developed kinetic model for describing the LF/LIF process. The information obtained in the experiment made it possible to clarify the values of the rate constants of the nitrobenzene dissociation.

Results of calculation of the absorption spectrum of phosphorus oxide (PO) corresponding to the A²Σ⁺ (v′ = 0) − X ²Π (v′′ = 0) transition are presented. The calculation is performed taking into account the perturbation of the v′ = 0 level of the A²Σ⁺ state by the v′ = 4 level of the b⁴Σ⁻ state. The efficiency of laser excitation of PO molecules is estimated as a function of the spectral parameters of the radiation. The time dependence of the efficiency of the excitation of PO fragments is calculated for a real atmosphere under two-pulse laser fragmentation of organophosphates for specified spectral linewidths Δλ and center positions λ₀.

The paper presents the results of calculating the absorption spectrum of a phosphorus monoxide (PO) molecule corresponding to the A 2 Σ + ( v ′ = 0 ) − X 2 Π ( v ′ ′ = 0 ) transition. The efficiency of excitation of the PO molecule is estimated as a function of the spectral parameters of the laser radiation. The positions of the fluorescence bands of PO are calculated. It is shown that the use of excitation wavelengths near the bandheads of the ( P 22 + Q 12 ) and P 12 branches of the A 2 Σ + ( v ′ = 0 ) − X 2 Π 3 / 2 ( v ′ ′ = 0 ) band provides spectral separation of the (0, 1) γ -band of the LIF of PO and vibrational-rotational Raman spectrum of atmospheric oxygen. The spectral responses of dimethyl methylphosphonate vapor in air under the action of KrF-laser radiation at a wavelength of 247.78 nm have been experimentally studied. In the wavelength range 252–260 nm, the (0, 1) γ -band of the LIF of PO-fragments and the vibrational-rotational band of spontaneous Raman scattering on oxygen molecules are unambiguously interpreted. It is shown that the results of calculations of the shape and position of the fluorescence spectra are in good agreement with the experimental data.

The paper presents the results of a numerical evaluation of limiting sensitivity of the method for detecting vapors of nitrocompounds in the atmosphere based on one-color laser fragmentation (LF)/laser-induced fluorescence (LIF) of NO fragments via A²Σ⁺ (v′ = 0) ← X²Π (v″ = 2) transition. The calculations were performed using the developed kinetic model of the one-color LF/LIF process under consideration. The calculations take into account the influence of ambient nitrogen dioxide as a limiter of the sensitivity of the method when operating in a real atmosphere. It is shown that if the nitrogen dioxide concentration in the atmosphere does not exceed a value of 10 ppb, the maximum detectable vapor concentrations of nitrobenzene and o-nitrotoluene are several ppb. It is also shown that the method of single-frequency one-color excitation usually used for the detection of nitrocompounds does not allow achieving the maximum efficiency of the LF/LIF process.

The vibrational and rotational terms of the \({{X}^{2}}\Pi \) and \({{A}^{2}}{{\Sigma }^{ + }}\) electronic states of the phosphorus oxide (PO) molecule are calculated, as well as the absorption spectrum corresponding to the \({{A}^{2}}{{\Sigma }^{ + }}({v}{\kern 1pt} ' = 0;J{\kern 1pt} ') - {{X}^{2}}\Pi ({v}{\kern 1pt} '' = 0;J{\kern 1pt} '')\) transition. The efficiency of laser excitation of PO molecules is estimated versus spectral parameters of the radiation.