Picosecond Coherent anti-Stokes Raman Spectroscopy Research Articles

Nitric oxide kinetics in the afterglow of a diffuse plasma filament

A suite of laser diagnostics is used to study kinetics of vibrational energy transfer and plasma chemical reactions in a nanosecond pulse, diffuse filament electric discharge and afterglow in N2 and dry air at 100 Torr. Laser-induced fluorescence of NO and two-photon absorption laser-induced fluorescence of O and N atoms are used to measure absolute, time-resolved number densities of these species after the discharge pulse, and picosecond coherent anti-Stokes Raman spectroscopy is used to measure time-resolved rotational temperature and ground electronic state N2(v = 0–4) vibrational level populations. The plasma filament diameter, determined from plasma emission and NO planar laser-induced fluorescence images, remains nearly constant after the discharge pulse, over a few hundred microseconds, and does not exhibit expansion on microsecond time scale. Peak temperature in the discharge and the afterglow is low, T ≈ 370 K, in spite of significant vibrational nonequilibrium, with peak N2 vibrational temperature of Tv ≈ 2000 K. Significant vibrational temperature rise in the afterglow is likely caused by the downward N2–N2 vibration–vibration (V–V) energy transfer. Simple kinetic modeling of time-resolved N, O, and NO number densities in the afterglow, on the time scale longer compared to relaxation and quenching time of excited species generated in the plasma, is in good agreement with the data. In nitrogen, the N atom density after the discharge pulse is controlled by three-body recombination and radial diffusion. In air, N, NO and O concentrations are dominated by the reverse Zel'dovich reaction, N + NO → N2 + O, and ozone formation reaction, O + O2 + M → O3 + M, respectively. The effect of vibrationally excited nitrogen molecules and excited N atoms on NO formation kinetics is estimated to be negligible. The results suggest that NO formation in the nanosecond pulse discharge is dominated by reactions of excited electronic states of nitrogen, occurring on microsecond time scale.

Picosecond CARS measurements of nitrogen vibrational loading and rotational/translational temperature in non-equilibrium discharges

Picosecond coherent anti-Stokes Raman spectroscopy (CARS) is used to study vibrational energy loading and relaxation kinetics in nitrogen and air ns pulsed non-equilibrium plasmas, in both plane-to-plane and pin-to-pin geometries. In 10 kHz repetitively pulsed plane-to-plane plasmas, up to ∼50% of coupled discharge power is found to load vibrations, in good agreement with a master equation kinetic model. In the pin-to-pin geometry, ∼40% of total discharge energy in a single pulse in air at 100 Torr is found to couple directly to nitrogen vibrations by electron impact, also in good agreement with model predictions. Post-discharge, the total quanta in vibrational levels v = 0–9 is found to increase by ∼60% in air and by a factor of ∼3 in nitrogen, respectively, a result in direct contrast to modelling results which predict the total number of quanta to be essentially constant until ultimately decaying by V–T relaxation and mass diffusion. More detailed comparison between experiment and model show that the vibrational distribution function (VDF) predicted by the model during, and directly after, the discharge pulse is in good agreement with that determined experimentally. However, for time delays exceeding ∼1 µs, the experimental VDF shows populations of vibrational levels v ⩾ 2 greatly exceeding modelling results, which predict their predominant decay due to net downward V–V transfer and corresponding increase in v = 1 population. This is at variance with the experimental results, which show a significant monotonic increase in the populations of levels v = 2–9 at t ∼ 1–10 µs after the discharge pulse, both in nitrogen and air, before gradually switching to relaxation at t ∼ 10–100 µs. It is concluded that a collisional process is likely feeding high vibrational levels at a rate which is comparable to the rate at which population of the high levels is lost due to net downward V–V energy transfer. A likely candidate for the source of additional vibrational quanta is quenching of metastable electronic states of nitrogen to highly excited vibrational levels of the ground electronic state. The effect of electronic-vibrational (E–V) coupling on time-resolved N2 vibrational populations and temperature, estimated using a phenomenological E–V energy transfer model, provides qualitative interpretation of the present experimental results.

Generation of Simplified Protein Raman Spectra Using Three-Color Picosecond Coherent Anti-Stokes Raman Spectroscopy

The well-known and prominent marker bands of aromatic amino acids in Raman spectra of protein and peptide films are revisited in the frequency and time domains using three-color picosecond coherent anti-Stokes Raman spectroscopy (CARS). We show here that control of the probe delay allows the narrow width/long lifetime states to be observed free not only from nonresonant background and fluorescence contamination but also free from the spectral congestion that arises from the complex background of spectrally broader (shorter lifetime) vibrational modes. The reasonable limits of detection obtained indicate that such CARS methods may be useful for quantitative analysis of protein composition.

Strong S1−S2 Vibronic Coupling and Enhanced Third Order Hyperpolarizability in the First Excited Singlet State of Diphenylhexatriene Studied by Time-Resolved CARS

A strong S1−S2 vibronic coupling effect is observed in the time-resolved coherent anti-Stokes Raman spectroscopy (CARS) spectra originating from the first excited singlet state of diphenylhexatriene. As determined from picosecond CARS measurements, the excited state spectrum appears on a subpicosecond time scale. An extremely high excited state hyperpolarizability |γ|excited state ≅ 3 ×10-31 is derived from a CARS line shape analysis and is attributed to the increased delocalization after excitation in accordance with semiempirical calculations of bond lengths. We observe two strongly frequency-broadened vibrations being upshifted with respect to the CC double bond stretching region of the ground state and assign them to the totally symmetric CC stretching motion of the chain. Both frequencies depend on the solvent polarizability, giving evidence of strong S1−S2 vibronic coupling in the lowest excited singlet states. We discuss a model of S1−S2 vibronic coupling via an asymmetric low frequency mode. According to this model a double-well potential for the respective vibrational coordinate is generated in the first excited singlet state, resulting in two frequencies originating from the same type of vibration.

Picosecond time-resolved CARS spectroscopy of a mixed excited singlet state of diphenylhexatriene

Time-resolved coherent antistokes Raman spectroscopy (CARS) spectra from an excited singlet state of diphenylhexatriene (DPH) are reported. As determined from the CARS measurements, the excited-state population increases on a sub-picosecond time scale after excitation in the 1A g–1B u absorption band. We observe two strongly upshifted and anomalously broadened resonances in the double-bond stretching region. According to our semi-empirical calculations of vibrational frequencies, they can be assigned to polyenic p-chain modes of the 1B u and 2A g states, respectively. Frequency shifts as well as broadenings are extremely solvent-dependent, indicating a strong influence on the excited-state conformations. Our time-resolved measurements support a mixed excited singlet state with drastically different geometries of the local minima.

Characteristics of picosecond coherent anti-Stokes Raman spectroscopy with mode-locked dye lasers in comparison with stimulated Raman gain

Time-resolved coherent anti-Stokes Raman scattering (CARS) has been performed with a double synchronously pumped mode-locked dye-laser system that is the same as had been used earlier for stimulated Raman gain (SRG) experiments; the CARS signal is also spectrally resolved. We present an extensive experimental and theoretical study of the CARS technique and compare the results with the characteristics of SRG. Experimental tests have been performed on the 656- and 991-cm−1 modes in liquid CS2 and C6H6, respectively. In particular, the influence of the pulse chirps on the time resolutions in CARS and SRG has been examined. SRG profits from oppositely chirped dye-layer pulses. In these circumstances, the observed time resolution in SRG is typically a factor of 3 better than in CARS. Also, when measured under off-resonance conditions, CARS decay curves exhibit a narrow and pronounced coherence spike that is symmetrical near zero delay: its appearance is discussed in detail. In addition, we establish the conditions for which the phenomena, of critical frequency mistuning and spectral narrowing in time-delayed CARS are observable.

Picosecond coherent anti-Stokes Raman spectroscopy of molecules in free jet expansions

By using time-resolved coherent Raman scattering, closely spaced vibrational resonances are simultaneously excited and subsequently monitored over long, nanosecond time intervals. Coherent superposition of molecular transitions leads to dramatic oscillations of the picosecond anti-Stokes scattering signals. Experimental data are presented for the nu(1) mode of CH(4) in supersonic expansions at low rotational temperatures (T(ROT) >/= 25). Our timedomain data are in good agreement with spectroscopic results. The corresponding frequency resolution of less, similar3 x10(-3) cm(-1) suggests that this method of Fourier-transform Raman spectroscopy may prove useful in high-resolution molecular spectroscopy.