Experimental results are reported on a mobile, stand-alone, solar-blind ultraviolet (UV) Raman lidar system for the stand-off detection and identification of liquid and solid targets at ranges of hundreds of meters. The lidar is a coaxial system capable of performing range-resolved measurements of gases and aerosols, as well as solids and liquids. The transmitter is a flash lamp pumped 30 Hz Nd:YAG laser with quadrupled output at 266 nm. The receiver subsystem is comprised of a 40 cm Cassegrain telescope, a holographic UV edge filter for suppressing the elastic channel, a 0.46 m Czerny–Turner spectrometer, and a time gated intensified charge-coupled device (CCD) detector. The rejection of elastic light scattering by the edge filter is better than one part in 105, while the transmittance 500 cm−1 to the red of the laser line is greater than 50%. Raman data are shown for selected solids, neat liquids, and mixtures down to the level of 1% volume ratio. On the basis of the strength of the Raman returns, a stand-off detection limit of ∼ 500 g/m2 for liquid spills of common solvents at the range of one half of a kilometer is possible.

The Mini-Raman Lidar System (MRLS) is a portable chemical sensor that combines the spectral fingerprinting of Raman spectroscopy with the principles of solar-blind ultraviolet lidar for short-range, noncontact detection and identification of unknown substances on surfaces. The MRLS has the potential to detect contaminant films several microns thick at distances of meters and bulk quantities of substances at distances of tens of meters. The signal acquisition time is less than 1 min. The device has application to those involved in emergency response, environmental remediation, and military reconnaissance who respond initially at the site of a chemical spill or attack.

The total differential Raman cross section of the symmetric vibrational mode of CS2 (652 cm−1) in liquid phase has been measured as a function of excitation wavelength from the visible to the ultraviolet. The resulting excitation profile shows a strong preresonance enhancement when the excitation wavelength is less than 300 nm. The cross section measured at 240 nm is about three orders-of-magnitude larger than the ν4 dependence for Raman scattering. The observed preresonant effect appears to be dominated by the B21(Σu+1)←Σg+1 transition. A minimum in the excitation profile occurs at a wavelength that is associated with the peak of the near-UV absorption band (∼320 nm). The observed dip in the profile is ascribable to a quantum interference between the B21(Σu+1) and the two Renner–Teller components, B21 and A21(Δu1). The transition from the ground state to the lower electronic state is electronically forbidden, but it becomes vibronically allowed due to the Renner–Teller interaction. This may be the first observation of Raman resonance de-enhancement due to the interference involving three excited states.

BNL has been developing a remote sensing technique for the detection of atmospheric pollutants based on the phenomenon of resonance Raman LIDAR that has also incorporated a number of new techniques/technologies designed to extend it`s performance envelope. When the excitation frequency approaches an allowed electronic transition of the molecule, an enormous enhancement of the inelastic scattering cross-section can occur, often up to 2 to 4 orders-of-magnitude, and is referred to as resonance Raman (RR), since the excitation frequency is in resonance with an allowed electronic transition. Exploitation of this enhancement along with new techniques such as pattern recognition algorithms to take advantage of the spectral fingerprint and a new laser frequency modulation technique designed to suppress broadband fluorescence, referred to as Frequency modulated Excitation Raman Spectroscopy (FreMERS) and recent developments in liquid edge filter technology, for suppression of the elastic channel, all help increase the overall performance of Raman LIDAR.

An analysis of the b ˜ 1 B 1 − a ˜ 1 A 1 electronic spectrum of methylene was first reported in 1966 by Herzberg and Johns.1 Since that time, numerous experimental and theoretical investigations have been carried out on this chemically important species. The two lowest singlet states in CH2 lie approximately 3000 and 12000 cm-1 above the ground X ˜ 3 B 1 state and correlate with a degenerate 1 Δ g state at the linear configuration of the molecule. This degeneracy leads to extreme complexity in the spectrum. The a ˜ and b ˜ states may be regarded as derived from a severe Renner-Teller effect in a linear singlet CH2 molecule and the resulting vibronic structure leads to a highly irregular band system, especially at energies near to that of the degenerate linear configuration, recently2 calculated to lie some 8800 cm-1 above the zero point level of the a ˜ state. Although the visible region of the b ˜ − a ˜ spectrum has been extensively studied3,4 since the original work of Herzberg and Johns, there have been no experiments reported in the near infrared region due to less efficient dye laser operation at these wavelengths.

The theory of unimolecular decomposition has played a central role in the history of chemical kinetics and molecular dynamics. Energy-dependent rate constants, product final state distributions, and photofragment excitation spectra have provided experimental tests for simple and complex unimolecular theories. The use of Doppler spectroscopy to measure correlated velocity and angular momentum distributions can frequently provide a more complete picture of the dissociation process than other, more integrated measurements. The 193 nm dissociation of cyanogen (NCCN) has been considered a classic case of a barrierless dissociation, well described by simple phase space theory, where all asymptotic product states that conserve total energy and angular momentum are considered equally likely.

The CN photofragments from the photodissociation of NCCN at 193 nm have been measured by high-resolution transient absorption spectroscopy. Doppler-broadened profiles of isolated rotational lines in the 2-0 and 3–1 vibrational bands of the CN AX transition were observed under collisionless conditions with a tunable, single-frequency Ti:sapphire ring laser. Analysis of the Dopple profiles reveals a vector correlation between the translation and rotation of CN photoproducts, with the angular momentum of the high rotational states increasingly perpendicular to the recoil velocity. After correction for vector correlations, the laboratory-frame scalar speed distribution of state-selected photoproducts can be determined. The mean squared laboratory velocity is directly related to the average internal energy of coincident CN fragments. The wings of the Doppler profiles indicate that the available energy for a pair of ground state CN photoproducts following 193 nm dissociation of NCCN at 295 K is 5300±150 cm−1, which includes the average vibrational energy of the parent molecules selected by the photolysis laser. Phase space theory with an optimized available energy of 5300 cm−1 produces laboratory speed distributions that are in qualitatively reasonable agreement with the kinetic energy measurements, but overestimate the total internal energy of the photofragments. The measurements are good enough to warrant comparison with more sophisticated models of unimolecular decomposition.

We report measurements of 2SIGMA symmetry rovibronic levels in the (v1 v2, v3) = (110)/(030) Fermi-resonant polyad and the 040 mu 2PI states of NCO. The levels were detected by the technique of stimulated emission pumping spectroscopy in a supersonic free jet expansion following excitation of rotational lines in the A(100) 2SIGMA+ -X(000) 2PI transition of the radical. Combining the new measurements and previously published data we have a complete set of experimental data for 2SIGMA and 2PI vibronic levels of X(v1v20) 2PI NCO for v1 less-than-or-equal-to 2, v2 less-than-or-equal-to 4 and for the (010) 2DELTA vibronic states. A fairly complete effective Hamiltonian is described and the parameters in it are fitted to a sample representative of all these data. We successfully fit the data to close to the experimental measurement accuracy and derive a unique set of harmonic and anharmonic vibrational terms. The results are interpreted in terms of the electronic structure and vibrational potential energy surface of the radical.

Recent results obtained using absorption techniques with continuous wave lasers to measure vibrational and electronic spectra of small free radicals are reported.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We report measurements of 2Σ symmetry rovibronic levels in the (υ 1, υ 2, υ 3)= (110)/(030) Fermi-resonant polyad and the 040 μ 2π states of NCO. The levels were detected by the technique of stimulated emission pumping spectroscopy in a supersonic free jet expansion following excitation of rotational lines in the Ã(100) 2Σ+−[Xtilde](000) 2π transition of the radical. Combining the new measurements and previously published data we have a complete set of experimental data for 2Σ and 2π vibronic levels of [Xtilde](v1v2 0) 2π NCO for v 1⪯2, v 2⪯4 and for the (010) 2Δ vibronic states. A fairly complete effective Hamiltonian is described and the parameters in it are fitted to a sample representative of all these data. We successfully fit the data to close to the experimental measurement accuracy and derive a unique set of harmonic and anharmonic vibrational terms. The results are interpreted in terms of the electronic structure and vibrational potential energy surface of the radical.

A Doppler-limited high resolution vibronic spectrum of the methylene (CH2) b̃ 1B1←ã 1A1 transition in the near-infrared wavelength region has been obtained using transient absorption techniques. The radical was produced by 308 nm excimer laser photolysis of ketene (CH2CO) in a flow system. The analysis of this spectrum confirms the reassignments of some previously observed bands as well as the presence of new vibronic bands predicted by a recent ab initio calculation [Green et al., J. Chem. Phys. 94, 118 (1991)]. We also measured the Doppler broadened profiles of low-J rotational lines of CH2 under collisionless conditions. From the line profile analysis, we find that the 308 nm photolysis of ketene produces fragments with isotropic and uncorrelated velocity and angular momentum distributions. The Doppler profiles also provide a view of the coincident product state distributions. The measured Doppler profiles are consistent with a rotational distribution of CO produced in coincidence with low energy CH2 states given by statistical phase space theory. The vibrationally excited coincident CO appears, however, to be produced about 2–3 times more efficiently than is predicted by various statistical theories. A frequency modulation (FM) technique has been adopted to improve the sensitivity of the transient absorption experiment. There is a significant improvement in the observed signal to noise ratio of the CH2 spectrum over the dual beam method previously used.

The observation of laser-induced fluorescence spectra of the jet-cooled HNCN free radical is reported. The HNCN free radical was generated by ArF laser photolysis of a mixture of NH3, C2N2 and N2 in the ratio 1 : 1 : 10 in the first stages of a supersonic free jet expansion. Evidence suggests that the mechanism of formation of HNCN under these conditions is CN + NH2→ HNCN + H. The lifetime of the excited 2A′ state is found to be 20 ± 5 ns. Molecular constants were obtained by a least-squares fit of the present and previously published data to an effective rotational Hamiltonian. Approximate vibrational intervals in the ground state of the radical were measured for the first time by dispersing the laser-induced fluorescence. The dispersed fluorescence spectra show activity in a bending vibration, assigned from its parallel rotational structure to the δ(HN—C—N) a″ mode. This is interpreted as evidence for a Renner–Teller effect in the radical.

The correlated angular distribution of translational and rotational motion of photofragments is probed by Doppler-broadened transient absorption line shapes. The one-photon nature of the probe process reduces the complexity of the analysis, while maintaining sensitivity to five bipolar moments of the translational and rotational angular distributions. The 193-nm photodissociation of C2H5SCN illustrates the method, with CN products detected with a titanium:sapphire ring laser probing the A2PI-X2SIGMA+ transition. The vector correlations in the high rotational states of CN indicate a direct dissociation following a transition of mixed symmetry at 193 nm: the recoil is preferentially but not exclusively parallel to a transition moment, and the perpendicular component is dominated by a rotation axis parallel to a C2H5SCN transition moment. The strong rotational excitation is generated by planar bending forces in a bent excited state. Lower rotational states are formed with a composite kinetic energy distribution, indicating an additional, slower, less highly polarized channel that partitions more energy into the unobserved C2H5S radical than does the direct channel. At 248 nm, a weaker absorption also generates CN photofragments with a preferentially parallel recoil, requiring a reassessment of the excited-state assignments in the alkyl thiocyanates.

We report the observation of stimulated emission pumping spectra in the NCO radical formed in a supersonic free jet expansion by the reaction between photolytically generated CN radicals and O2. The spectra give rotationally resolved information on high lying vibrational levels that are difficult or impossible to detect by conventional single photon spectroscopic techniques. These new data provide detailed insight into the Renner-Teller, spin-orbit and Fermi-resonance coupling in the molecule. They also provide a solid basis for future state- selected chemical and dynamical studies involving this important radical species.

Laser induced fluorescence measurements of NCO formed in the reaction CN+O2→NCO+O in a free jet expansion are reported. The rotational temperature achieved in the experiments was typically 10–15 K and absorption from vibrationally excited levels was very much reduced in intensity. Two bands in the ÖX̃ system, the v1, v2, v3=200–000 and 120–000, of the radical were rotationally resolved for the first time, and wavelength resolved laser excited fluorescence measurements have been used to characterize many ground state vibronic energy levels below 6000 cm−1 in energy. Stimulated emission pumping experiments were carried out on vibronic levels associated with the ν3 vibration with 1≤v3≤3 in the electronic ground state. Harmonic and anharmonic constants were determined, together with the spin–orbit coupling parameters and rotational constants for these levels.

We report extensive measurements of rovibronic levels in the 020/100 and 040/120/200 Fermi-resonant polyads of NCO. The levels were accessed by the technique of stimulated emission pumping spectroscopy in a supersonic free jet expansion following excitation of bands in the à 2Σ+−X̃ 2Π spectrum of the radical. The data were analyzed in terms of an effective Hamiltonian which explicitly included reference to all the possible vibronic levels of 2Π symmetry in the polyad under consideration. The effects of levels outside these were treated by perturbation theory. The model was successful in fitting the data close to the experimental measurement precision and the resulting parameters were interpreted in terms of the harmonic and anharmonic vibrational terms. The effective Renner–Teller coupling parameter in NCO varied with vibrational level; however, parameters describing the Fermi-resonance interaction were found to be constant for all the levels investigated.