Laser-induced Fluorescence Excitation Research Articles

The effects of site asymmetry on near-degenerate state-to-state vibronic mixing in flexible bichromophores.

Laser-induced fluorescence excitation and dispersed fluorescence spectra of a model flexible bichromophore, 1,1-diphenylethane (DPE), have been recorded under jet-cooled conditions in the gas phase in the region near the first pair of near-degenerate excited states (S1 and S2). The S1 and S2 origin transitions have been identified at 37 397 and 37 510 cm-1, a splitting of 113 cm-1. This splitting is four times smaller than the excitonic splitting calculated by ab initio methods at the EOM-CCSD/cc-pVDZ level of theory (410 cm-1), which necessarily relies on the Born-Oppenheimer approximation. Dispersed fluorescence spectra provide a state-to-state picture of the vibronic coupling. These results are compared with the results of a multimode vibronic coupling model capable of treating chromophores in asymmetric environments. This model was used to predict the splitting between S1 and S2 origins close to the experiment, reduced from its pure excitonic value by Franck-Condon quenching. Quantitative accuracy is achieved by the model, lending insight into the state-to-state mixing that occurs between individual S1 and S2 vibronic levels. The S2 origin is determined to be mixed with S1(v) levels by two mechanisms common to internal conversion in almost any setting; namely, (i) mixing involving near-degenerate levels with large vibrational quantum number changes that are not governed by Δv = 1 Herzberg-Teller (HT) selection rules, and (ii) mixing with levels with larger energy gaps that do follow these selection rules. In DPE, the asymmetric ring flapping vibrational mode R¯ dominates the HT coupling.

Laser-induced fluorescence spectroscopy of jet-cooled WO in 18, 900–23, 500 cm−1

The laser-induced fluorescence (LIF) excitation spectra of WO molecule have been recorded in the energy range of 18,900–23,500 cm−1 using laser ablation of tungsten target and reaction with free jet expansion of oxygen. Totally 63 vibronic transition bands were observed, and 60 bands were grouped into 10 electronic transition progressions, where 8 electronic transition progressions have the X0+ state as the lower state, as well as 2 progressions have the X1 state as the lower state. The electronic transition systems [21.5]0+–X0+ and [23.3]1–X0+ were newly identified by LIF spectroscopy. The molecular constants, including rotational constant, vibrational frequency, vibrational anharmonic constant, in the electronic excited states were obtained through rotational analysis of the spectra. In addition, the fluorescence lifetimes of the vibronic states were measured under the collision-free condition by exponentially fitting the fluorescence decay.

Soot inception in laminar coflow diffusion flames

This paper focuses on the soot inception region in laminar coflow diffusion flames of methane and ethylene stabilised on the Yale diffusion burner. Earlier studies of these flames have focused on the downstream regions where soot has already developed. Laser-induced fluorescence (LIF) and elastic scattering measurements from 266 nm excitation are combined with laser-induced incandescence (LII) excited at 1064 nm. The structure and evolution of the soot precursor particles are characterised using the LIF intensity, decay time and the relative spectral emission in the ultraviolet and visible. The LIF decay times indicate that the majority of 266 nm excited LIF originates from nanostructures rather than gas phase polycyclic aromatic hydrocarbons (PAH). A similarity in the particle evolution for the low and high sooting flames in the upstream regions is found, indicating a general transition towards larger structures with more aromatic features as the nanostructures advect downstream in the fuel rich pyrolytic conditions. Higher nanostructure concentrations are found to precede the higher soot volume fractions (SVF) found in fuel rich sootier flames, although not proportionally, suggesting that surface growth strongly contributes to SVF in the high sooting flames. In the heavier sooting flames the majority of particle formation shifts from the centreline to the wings at the outer edges of the flame closer to stoichiometry. Particle formation in the wings of the flames occurs in the presence of oxygen and higher temperatures, resulting in particles with spectroscopic properties resembling those formed toward the oxidiser side of counter-flow diffusion flames. In the heavier sooting flames, particles produced in the wings appear to mix with particles formed along the centreline of the flame at the flame tip, resulting in a broad range of nanostructures and soot occurring in this region.

Vibronic spectroscopy of methyl anthranilate and its water complex: hydrogen atom dislocation in the excited state.

Laser-induced fluorescence (LIF) excitation, dispersed fluorescence (DFL), UV-UV-hole burning, and UV-depletion spectra have been collected on methyl anthranilate (MA, methyl 2-aminobenzoate) and its water-containing complex (MA-H2O), under jet-cooled conditions in the gas phase. As a close structural analog of a sunscreen agent, MA has a strong absorption due to the S0-S1 transition that begins in the UV-A region, with the electronic origin at 28 852 cm-1 (346.6 nm). Unlike most sunscreens that have fast non-radiative pathways back to the ground state, MA fluoresces efficiently, with an excited state lifetime of 27 ns. Relative to methyl benzoate, inter-system crossing to the triplet manifold is shut off in MA by the strong intramolecular NHO[double bond, length as m-dash]C H-bond, which shifts the 3nπ* state well above the 1ππ* S1 state. Single vibronic level DFL spectra are used to obtain a near-complete assignment of the vibronic structure in the excited state. Much of the vibrational structure in the excitation spectrum is Franck-Condon activity due to three in-plane vibrations that modulate the distance between the NH2 and CO2Me groups, ν33 (421 cm-1), ν34 (366 cm-1), and ν36 (179 cm-1). Based on the close correspondence between experiment and theory at the TD-DFT B3LYP-D3BJ/def2TZVP level of theory, the major structural changes associated with electronic excitation are evaluated, leading to the conclusion that the major motion is a reorientation and constriction of the 6-membered H-bonded ring closed by the intramolecular NHO[double bond, length as m-dash]C H-bond. This leads to a shortening of the NHO[double bond, length as m-dash]C H-bond distance from 1.926 Å to 1.723 Å, equivalent to about a 25% reduction in the HO distance compared to full H-atom transfer. As a result, the excited state process near the S1 origin is a hydrogen atom dislocation that is brought about primarily by heavy atom motion, since the shortened H-bond distance results from extensive heavy-atom motion, with only a 0.03 Å increase in the NH bond length relative to its ground state value.

Laser-induced fluorescence spectroscopic study of the [21.2]0+–X0+ and [22.2]0+–X0+ electronic transitions of tungsten monoxide

The laser-induced fluorescence (LIF) excitation spectra of jet-cooled WO molecule have been investigated in the range of 21,000–23,200 cm−1, and two electronic band systems have been observed for the first time and labeled as the [21.2]0+–X0+ and [22.2]0+–X0+ transitions. The vibrational and rotational constants of these two excited electronic states have been derived by rotational analysis of the spectra. The tungsten isotopic structures of these two excited states are clearly resolved, and the analysis of LIF spectra reveals that the isotopic shifts are due to the strong interstate perturbations between different vibronic states. Based on the ab initio calculation results by Ram et al., the primarily attributed configuration of these two newly identified states was discussed. In addition, the lifetimes of the observed vibronic states were measured under the collision-free condition.

Eye-Safe Wavelength Laser Fluorimeter for Noncontact Monitoring the Ecological Condition of Objects in Natural Environment

Presently, a problem of ecological monitoring of natural environment is critical.Laser methods are the most powerful to provide noncontact and remote ecological monitoring of natural environment. Among laser monitoring methods, laser fluorescent ones are of strong interest.Today, there are laser fluorescent devices designed to monitor ecological condition of various objects in natural environment. To produce laser-induced fluorescence excitation, laser fluorescent devices use a variety of radiation sources in the spectral band from 226 to 635 nm.However, to have an advanced laser fluorescent device for noncontact ecological monitoring of natural environment, devices to be created should be multifunctional and use an eye-safe wavelength for fluorescence excitation.A fluorescence excitation wavelength of 355 nm (the third harmonic of a YAG: Nd laser) appears to be promising. This wavelength is eye-safe and can be used to monitor ecological condition of a large number of nature-made objects.The paper conducts a capability analysis of noncontact laser fluorescent monitoring of ecological condition of natural environment using the eye-safe fluorescence excitation wavelength of 355 nm. Describes a laser fluorimeter mock-up and presents experimental results of laser-induced fluorescence spectra of nature-made objects.The experimental studies have shown that the noncontact laser fluorimeter using the eye-safe fluorescence excitation wavelength of 355nm potentially allows us to control the ecological condition of a large number of nature-made objects.The fluorescence intensities in the certain spectral ranges (for example, within the spectral range of ~ 425 – 450 nm) and a spectrum profile of plant fluorescence (in the spectral range of 670 - 750 nm) can be used as the controlled variables.

Conformational explosion: Understanding the complexity of short chain para-dialkylbenzene potential energy surfaces.

The single-conformation ultraviolet and infrared spectroscopy of three short-chain para-dialkylbenzenes (para-diethylbenzene, para-dipropylbenzene, and para-dibutylbenzene) is reported for the jet-cooled, isolated molecules. The present study builds off previous work on single-chain n-alkylbenzenes, where an anharmonic local mode Hamiltonian method was developed to account for stretch-bend Fermi resonance in the alkyl CH stretch region [D. P. Tabor et al., J. Chem. Phys. 144, 224310 (2016)]. The jet-cooled molecules are interrogated using laser-induced fluorescence (LIF) excitation, fluorescence dip infrared spectroscopy, and dispersed fluorescence. The LIF spectra in the S1 ← S0 origin region show a dramatic increase in the number of resolved transitions with increasing length of the alkyl chains, reflecting an explosion in the number of unique low-energy conformations formed when two independent alkyl chains are present. Since the barriers to isomerization of the alkyl chain are similar in size, this results in an "egg carton" shaped potential energy surface. A combination of electronic frequency shift and alkyl CH stretch infrared spectra is used to generate a consistent set of conformational assignments. Using these experimental techniques in conjunction with computational methods, subsets of origin transitions in the LIF excitation spectrum can be classified into different conformational families. Two conformations are resolved in para-diethylbenzene, seven in para-dipropylbenzene, and about nineteen in para-dibutylbenzene. These chains are largely independent of each other as there are no new single-chain conformations induced by the presence of a second chain. A cursory LIF excitation scan of para-dioctylbenzene shows a broad congested spectrum at frequencies consistent with interactions of alkyl chains with the phenyl π cloud.

Conformational Preference Determined by C-H···π Interaction of an O-H···O Hydrogen-Bonded Binary Complex of p-Fluorophenol with 2,5-Dihydrofuran: A Laser-Induced Fluorescence Spectroscopy Study.

Conformational preferences of a binary hydrogen-bonded complex between p-fluorophenol (pFP) and 2,5-dihydrofuran (DHF) have been studied by means of laser induced fluorescence (LIF) spectroscopy in a supersonic jet expansion. Calculation predicts two major conformers for this complex, one having a nearly linear geometry in which the two molecular moieties are bound only by an O-H···O H-bond, but in the other an additional C-H···π type interaction between an ortho C-H group of pFP and ethylene group of DHF contributes to the binding stabilization and results in a folded geometry for the complex with respect to a global view, although the H-bond angle of the latter is relatively larger. This prediction is realized experimentally by identifying transitions corresponding to the two discrete conformers in a vibrationally resolved LIF excitation spectrum of the complex, and the red shifts of S1-S0 origin band of pFP moiety of the two conformers are 542 and 659 cm-1, respectively. The assignments are corroborated by means of dispersed fluorescence (DF) spectroscopy. In comparison, the LIF spectral bands for the pFP-tetrahydrofuran complex can be corresponded to only one conformer, whose S1-S0 origin transition shows a red shift (563 cm-1) somewhat similar to the linear conformer of pFP-DHF complex. Such similarities in spectral shifting behavior is consistent with the predictions of electronic structure calculations. The DF spectra also reveal that the energy threshold and pathways of vibrational dynamics in S1 of the two conformers show different behavior. Excitation to 6a1 level of pFP moiety of the folded conformer displays signatures of restricted intramolecular vibrational energy redistribution (IVR), whereas the linear form displays the emission feature for dissipative IVR.

Laser-Induced Fluorescence Investigations for Temperature Measurements in a Carbon Dioxide Flow

The objective of the presented work was the investigation of two-photon laser-induced fluorescence excitation spectra of carbon monoxide (CO) molecules (B-X) in a carbon dioxide () flow with an enthalpy of . The flow was generated with an arcjet-driven plasma wind tunnel. For validation, different stoichiometric premixed flat flames and a low-pressure cold-gas cell filled with CO have been examined. For the excitation of the Q-branch of the transition, the laser was tuned from 230.04 to 230.11 nm, with a step width of 0.0005 nm. Simulated spectra were calculated in advance with a simulation tool called NOCO-Spectra and showed good agreement with the experimental data. Finally, the rotational temperature of CO in the flow was evaluated by comparison of the CO-excitation spectra and the calculated spectra using a correlation automated rotational fitting procedure.

Effect of Facial Encumbrance on Excimer Formation and Charge Resonance Stabilization in Model Bichromophoric Assemblies

Excimer formation and charge resonance stabilization in covalently linked bichromophoric systems with flexible spacers are important processes relevant to biochemistry and functional materials. Requiring a π-stacked cofacial arrangement of a pair of aromatic molecules at a van der Waals contact, the underlying geometrical reorganization that accompanies these events continues to be debated. Here we use a variety of methods including two-color resonant two-photon ionization spectroscopy (2CR2PI), ion yield measurements, hole-burning spectroscopy (HB), and laser-induced fluorescence (LIF) excitation and emission spectroscopy to compare the gas-phase spectroscopy and dynamics of the van der Waals dimers of fluorene, 9-methylfluorene (MF), and 9,9′-dimethylfluorene (F1). The goal of this work is to probe the influence of methyl substitution on the dynamics of excimer formation and charge resonance (CR) stabilization. The fluorene dimer, (F)2, displays lifetime broadened electronic spectra and the dominance of...

Experimental Spectroscopic Studies of Carbon Monoxide (CO) Fluorescence at High Temperatures and Pressures

Two-photon excitation laser-induced fluorescence of carbon monoxide (CO-LIF) is investigated experimentally in order to determine the applicability of this technique for imaging CO concentration in aeronautical combustors. Experiments are carried out in a high temperature, high-pressure test cell, and in a laminar premixed CH4/air flame. Influence of temperature and pressure on CO-LIF spectra intensity and shape is reported. The experimental results show that as pressure increases, the CO-LIF excitation spectrum becomes asymmetric. Additionally, the spectrum strongly shifts to the red with a quadratic dependence of the collisional shift upon pressure, which is different from the classical behavior where the collisional shift is proportional to pressure. Moreover, pressure line broadening cannot be reproduced by a Lorenztian profile in the temperature range investigated here (300-1750 K) and, therefore, an alternative line shape is suggested.

Identifying the first folded alkylbenzene via ultraviolet, infrared, and Raman spectroscopy of pentylbenzene through decylbenzene.

The conformational preferences of pentyl- through decylbenzene are studied under jet-cooled conditions in the gas phase. Laser-induced fluorescence excitation spectra, fluorescence-dip infrared spectra in the alkyl CH stretch region, and Raman spectra are combined to provide assignments for the observed conformers. Density functional theory calculations at the B3LYP-D3BJ/def2TZVP level of theory provide relative energies and normal mode vibrations that serve as inputs for an anharmonic local mode theory introduced in earlier work on alkylbenzenes with n = 2-4. This model explicitly includes anharmonic mixing of the CH stretch modes with the overtones of scissors/bend modes of the CH2 and CH3 groups in the alkyl chain, and is used to assign and interpret the single-conformation IR spectra. In octylbenzene, a pair of LIF transitions shifted -92 and -78 cm-1 from the all-trans electronic origin have unique alkyl CH stretch transitions that are fit by the local model to a g1g3g4 conformation in which the alkyl chain folds back over the aromatic ring π cloud. Its calculated energy is only 1.0 kJ mol-1 above the all-trans global minimum. This fold is at an alkyl chain length less than half that of the pure alkanes (n = 18), consistent with a smaller energy cost for the g1 dihedral and the increased dispersive interaction of the chain with the π cloud. Local site frequencies for the entire set of conformers from the local mode model show 'edge effects' that raise the site frequencies of CH2(1) and CH2(2) due to the phenyl ring and CH2(n - 1) due to the methyl group. The g1g3g4 conformer also shows local sites shifted up in frequency at CH2(3) and CH2(6) due to interaction with the π cloud.

Investigation of transient ignition processes in a model scramjet pilot cavity using simultaneous 100 kHz formaldehyde planar laser-induced fluorescence and CH* chemiluminescence imaging

Ignition processes in scramjet pilot cavities are highly transient events that are influenced by factors including freestream Mach number and inlet geometry, turbulence intensity, cavity geometry, ignition source, and fueling composition and flow rate. In particular, the location of the flame kernel and associated propagation rate of the flame front throughout the cavity can significantly influence the end state of the ignition process. In this work formaldehyde (CH2O) was used as a flame marker to track ignition progress in a plane throughout the span-wise width of the cavity, while chemiluminescence imaging provided path-integrated flame location along the span-wise and axial directions. Planar laser-induced fluorescence (PLIF) excitation utilized the 355nm frequency-tripled output of an Nd:YAG burst-mode laser operating at 50–100kHz over 10ms with available pulse energy up to 80mJ. Simultaneous CH* chemiluminescence imaging from the top of the cavity was obtained with a high-speed complementary metal-oxide semiconductor camera. A freestream Mach number of 2 with ethylene fuel rates from 55–90 standard liters per minute were examined along with two different ignition sources: a spark discharge and pulse detonator. The resulting formaldehyde PLIF and chemiluminescence images indicate a strong correlation between fueling rate and the delay between the onset of ignition and stable combustion. More importantly, the span-wise propagation rate and structure of the flame front is highly dependent on the fueling rate, burning region, and ignition source.

Supersonic Jet Spectroscopy and Density Functional Theory Study of Isomeric Diazines: 1,4- and 1,8-Diazatriphenylene. Why Do They Differ So Deeply?

We report on laser-induced fluorescence excitation and dispersed fluorescence spectra of two isomeric compounds: 1,4- and 1,8-diazatriphenylene (1,4- and 1,8-DAT) isolated in supersonic molecular jets, and their 1:1 complexes with protic solvents. We found that the ground and excited state vibronic patterns of bare 1,4-DAT differ significantly from those of 1,8-DAT, and those of the complexes of both isomers. A marked activity of several out-of-plane vibrations in 1,4-DAT and the symptoms of the distortion of the S1 excited molecule were diagnosed from the vibronic spectra, whereas planar structures were predicted for 1,8-DAT in S0 and S1 states. An anharmonic double-minimum potential along an out-of-plane coordinate has been derived and used to predict higher overtones of the S1 state vibration at 113 cm-1. Large enhancement of fluorescence was observed upon formation of 1:1 complexes of 1,4-DAT with water or methanol, which is explained in terms of an increased separation of interacting (n,π*) and (π,π*) electronic states in the H-bonded complexes, and/or a suppression of the intersystem crossing process.

Electronic spectrum of jet cooled SiCN.

We have generated SiCN in a supersonic free expansion, and measured the laser induced fluorescence (LIF) spectrum. Prior to the experiments, ab initio calculations were carried out to obtain the information necessary for searching for the LIF signals. In addition to the X̃ 2Π state, the optimized structures of three excited states, 2Δ, 2Σ+, and 2Σ-, have been obtained. Guided by the predictions, the LIF excitation spectrum of SiCN was observed in the UV region. The rotational structure of the 000 band with the origin, 29 261.639 cm-1, indicated that the electronic transition is à 2Δ-X̃ 2Π. The spin-orbit (SO) constants of the X̃ 2Π and à 2Δ states were determined to be 140.824 and 4.944 cm-1, respectively. In the à 2Δ state, the Fermi resonance between the (0, 20, 0) 2Δ and (0, 00, 1) 2Δ vibronic levels was identified. The molecular constants of the X̃ 2Π state were determined through the simultaneous analysis of the combination differences derived from the present LIF data with the previously reported rotational transitions. The spectroscopic parameters of the à 2Δ state were also obtained from the analysis where the constants of the X̃ 2Π state, derived above, were fixed at those values.

Rotational profiles in the excitation spectrum recorded for the B31(53P1) ⿿ X10+(51S0) transition in CdNe van der Waals complex

We report rotational profiles in the laser induced fluorescence excitation spectrum recorded for the B31(53P1)⿿X10+(51S0) transition in CdNe van der Waals complex. A method of analyzing the rotational structure of overlapping vibrational bands with isotopic spectral contributions is discussed. Rotational characteristics BϿ and DϿ for the Ͽ⿲=0, 1, 2 and Ͽ⿳=0 are reported.

Formaldehyde preparation methods for pressure and temperature dependent laser-induced fluorescence measurements.

Formaldehyde is an excellent tracer for the early phase of ignition of hydrocarbon fuels and can be used, e.g., for characterization of single droplet ignition. However, due to its fast thermal decomposition at elevated temperatures and pressures, the determination of concentration fields from laser-induced fluorescence (LIF) measurements is difficult. In this paper, we address LIF measurements of this important combustion intermediate using a calibration cell. Here, formaldehyde is created from evaporation of paraformaldehyde. We discuss three setups for preparation of formaldehyde/air mixtures with respect to their usability for well-defined heating of formaldehyde/air mixtures. The "basic setup" uses a resist heater around the measurement cell for investigation of formaldehyde near vacuum conditions or formaldehyde/air samples after sequential admixing of air. The second setup, described for the first time in detail here, takes advantage of a constant flow formaldehyde/air regime which uses preheated air to reduce the necessary time for gas heating. We used the constant flow system to measure new pressure dependent LIF excitation spectra in the 343 nm spectral region (41 (4) absorption band of formaldehyde). The third setup, based on a novel concept for fast gas heating via excitation of SF6 (chemically inert gas) using a TEA (transverse excitation at atmospheric pressure) CO2 laser, allows to further minimize both gas heating time and thermal decomposition. Here, an admixture of CO2 is served for real time temperature measurement based on Raman scattering. The applicability of the fast laser heating system has been demonstrated with gas mixtures of SF6 + air, SF6 + N2, as well as SF6 + N2 + CO2 at 1 bar total pressure.

Laser spectroscopy of the Ã(2)Σ(+)-X̃(2)Πi band system of l-SiC3H.

The Ã(2)Σ(+)-X̃(2)Πi band system of l-SiC3H in the region 14,700-16,300 cm(-1) was re-investigated by laser induced fluorescence (LIF) and fluorescence depletion spectroscopy. Rotational analyses were made for three intense bands 0(0)(0), 4(0)(1), and 6(0)(1)7(0)(1) by observing high-resolution LIF excitation spectra. The determined rotational constants demonstrate that SiC3H is linear in the à state, as is the case in the X̃ state, and the observed band types are consistent with the vibrational assignments. The ν3(″) (C1-C2 stretch) level was identified in a newly observed dispersed fluorescence spectrum from the zero-vibrational level of the à state.

Simultaneous two-dimensional measurement of fuel–air ratio and temperature in a direct-injection spark-ignition engine using a new tracer-pair laser-induced fluorescence technique

A planar tracer laser–induced fluorescence technique using a new tracer pair is introduced and has been utilized for the simultaneous detection of the fuel–air ratio and the temperature to characterize mixture formation inside a direct-injection spark-ignition engine. The new tracer pair consists of triethylamine and 3-pentanone, which are simultaneously added to the surrogate fuel isooctane. Triethylamine was used for the determination of the fuel–air ratio. The temperature distribution was evaluated with the two-line excitation laser-induced fluorescence technique using 3-pentanone. Prior to the internal combustion engine measurements, the tracer mixture was first investigated in a flow cell to demonstrate the spectral separability of the fluorescence signals and for the extension of the calibration data basis. As a first application, the tracer pair was applied inside a direct-injection spark-ignition engine operated with a split-injection scheme. For the late second injection, the fuel and temperature stratification was studied during the compression stroke. The measured temperature drop due to evaporative cooling effects was found to be about 100 K in the averaged data and up to 125 K in the single-shot measurements. The relation between fuel-rich areas and strong temperature drop can clearly be seen in the results. Furthermore, the suitability of the tracer pair to resolve cyclic variations is visualized.

Collisional Relaxation of NCN: An Experimental Study with Laser-Induced Fluorescence

Abstract Collisional relaxation of NCN produced by photolysis of NCN3 at a wavelength of 248 nm has been investigated with laser-induced fluorescence (LIF) in a temperature range of 240–293 K and a pressure range of 10–800 mbar with different bath gases (He, Ne, Ar, Kr, H2, N2, O2, and N2O). LIF excitation spectra were recorded, and LIF intensity-time profiles experimentally obtained at an excitation wavelength of 329.01 nm have been fitted with biexponential functions. The pressure and bath gas dependence of the rate coefficients obtained was rationalized in terms of a simple Landau–Teller/Schwartz–Slawsky–Herzfeld model, and notable influences of vibrational energy transfer to the rate of the overall relaxation process have been found. Evidence was obtained for a two-channel vibrational relaxation in NCN.