Index of content:
Volume 97, Issue 8, 15 October 1992

High resolution laser spectroscopy of free radical‐inert gas complexes: C_{5}H_{5}⋅He, C_{5}H_{5}⋅He_{2}, C_{5}H_{5}⋅Ne, and CH_{3}–C_{5}H_{4}⋅He_{2}
View Description Hide DescriptionSeveral inert‐gas complexes of the free radical cyclopentadienyl and its methyl derivative (C_{5}H_{5}⋅He, C_{5}H_{5}⋅Ne, C_{5}H_{5}⋅He_{2}, and CH_{3}–C_{5}H_{4}⋅He_{2}, and CH_{3}–C_{5}H_{4}⋅He) have been studied at low temperature via their laser induced fluorescencespectra in a supersonic free jet expansion. The rotational structures of the near‐uv electronic transitions of the radicals have been resolved and analyzed. These analyses lead to precisely defined geometries for the radicals. In contrast to inert‐gas, closed‐shell aromatic complexes, the inert‐gas radical ring bond lengths are found to be strongly dependent upon electronic state and methyl substitution. Additional molecular parameters were measured in the complexes including the electronic angular momentum, the (2,2) interaction parameter for the C_{5}H_{5} complexes, and for the methyl derivative, the barrier to methyl rotation. These are compared to their values in the uncomplexed radicals.

Intramolecular dynamics. III. Theoretical studies of the CH overtone spectra for benzene
View Description Hide DescriptionThe CH overtone spectra for v _{CH}=1, 2, and 3 for an intermediate sized molecule, benzene, and the related intramolecular vibrational dynamics are treated theoretically. For this purpose, an artificial intelligence (AI) search technique is employed, using the evaluation function developed in Part II. The curvilinear local‐normal mode coordinate system discussed in Part I is also used. The main features of the theoretical spectra are in reasonable accord with those of the experimental ones. Results reflecting the important role of the symmetry of molecules even in their high energy states are described.

Spectroscopy and nuclear dynamics of tetracene–rare‐gas heteroclusters
View Description Hide DescriptionIn this paper we report on the electronic spectroscopy of mass‐resolved tetracene⋅Ar_{ n } (n=1–26) and tetracene⋅Kr_{ n } (n=1–14) heteroclusters, utilizing two‐photon, two‐color near‐threshold ionization in conjunction with mass‐spectrometric detection. The spectra of the S _{0} → S _{1} transition and the ionization threshold of these heteroclusters were monitored. The structured spectral features of the S _{0} → S _{1} transition of small‐ and medium‐sized (n=1–8) heteroclusters were attributed to the electronic origins of structural isomers and to their intermolecular vibrations. The S _{0} → S _{1} spectra of large (n≥9) heteroclusters are broad and were assigned to inhomogeneous broadening due to the coexistence of isomers, with the spectral feature(s) of each distinct isomer being homogeneously broadened. Isomer‐specific inhomogeneous line broadening was interrogated by the observation of isomer‐specific ionization potentials for medium‐sized (n=6–7) heteroclusters and of the dependence of the relative intensities of the spectral features on the conditions of the supersonic expansion.
The ionization thresholds of the tetracene⋅A_{ n } (A=Ar,Kr) reveal a linear (or superlinear) n dependence, being qualitatively different from the sublinear n dependence of the spectral shifts. These different patterns of the size dependence can be traced to the different intermolecular interactions which govern excitation and ionization and to the difference in the charge distribution in S _{0} and in the positive ion. The experimental spectroscopic data for the spectral shifts and the spectral linewidths were simulated in terms of the first and second moments of the classical line shape, which were obtained from Monte Carlo(MC) constant temperature simulations, in conjunction with a two‐parameter fit of the excited‐state tetracene–rare‐gas potential. The Monte Carlo simulations of the structural fluctuation parameters identified several isomerization phenomena, i.e., correlated restricted and unrestricted surface motion, adcluster isomerization,surface melting and side crossing, and characterized the size dependence of the temperature onsets of these processes for small and medium sized n=2–20 clusters. These isomerization processes could not be interrogated by the investigation of the size dependence of the spectral shifts and linewidths. The size dependence and the isomer specificity of the spectral shifts are well accounted for by the MC simulations. The homogeneous spectral linewidths of small (n<8) clusters pertain to the spectroscopy of ‘‘static’’ isomers, while the line broadening of large (n≥20) clusters manifests inhomogeneous line broadening due to the coexistence of wetting and nonwetting isomers. The temperature dependence of the spectral shifts and inhomogeneous linewidths of large (n≥20) clusters provides means for internal cluster thermometry.

Dipole moment differences of nonpolar dyes in polymeric matrices: Stark effect and photochemical hole burning. I
View Description Hide DescriptionThe difference of the induced dipole moments Δμ^{ind} between the ground and excited state of (centro‐)symmetric dye molecules (tetraphenylporphin and tetrapropylporphycene) embedded in different nonpolar and polar polymers is investigated. The technique of photochemicalhole burning combined with Stark effect measurements was used to determine the values as a function of the burning frequency. For almost all systems an increase of Δμ^{ind} from higher to lower optical frequencies was found. A plot of the <Δμ^{ind}≳ values measured at the absorption band maximum vs the low‐temperature values of the dielectric constant ε shows a clear correlation between the dipole moment difference and the low‐temperature ε values over the whole investigated range which is accessible with commercially available polymers. In order to evaluate the data the dielectric constant ε of each sample was determined in a series of separate experiments as a function of frequency and temperature.

Far‐infrared spectra of diatomic polar molecules in simple liquids: Accounting for the anisotropic interaction
View Description Hide DescriptionA theoretical expression giving the far‐infrared spectrum of a dilute solution of diatomic polar molecules in a nonpolar liquid solvent is derived in order to analyze the influence on this spectrum from the J=1 and J=2 components of the Legendre polynomial expansion of the anisotropic solute–solvent potential. A non‐Markovian spectral theory incorporating finite correlation of the interaction, mixing effects between rotational lines and quantum intermolecular potential correlation functions is used. This theory allows one to analyze the influence of each anisotropic part of the interaction in terms of a very reduced set of parameters: the strength and the width of its time correlation function. The zero and finite frequency components of the J=2 contribution are also studied. In particular, we show that the use of quantum potential correlation functions gives a zero frequency component not only in the width, but also in the shift of the different rotational lines. Numerical results for DCl, HCl, and HF in SF_{6} liquid at 273 K are obtained.

Rotational spectra, structure, Kr‐83 nuclear quadrupole coupling constant, and the dipole moment of the Kr‐benzene dimer
View Description Hide DescriptionRotational spectra were observed for the Kr‐82, Kr‐83, Kr‐84, and Kr‐86 isotopic species of the Kr‐benzene dimer with the Mark II Balle‐Flygare Fourier transformmicrowave spectrometer. The spectra are those of a symmetric top, with the Kr on the symmetry axis of the benzene. In the most abundant Kr‐84 benzene dimer the rotational constantsB _{0}, D _{ J }, and D _{ JK } were found to be 795.6821(1) MHz, 1.315(1) kHz, and 7.895(4) kHz, respectively. R, the Kr to benzene center‐of‐mass distance, is 3.663 Å and the zero‐point vibrational bending of the benzene with respect to R gives an average angular displacement of 16.6°. The Kr‐83 quadrupole coupling constant was determined experimentally to be −5.201(5) MHz and the dipole moment, 0.136(2) D. A theoretical prediction of these electrically induced properties gives results for a distributed multipole model which are superior to those using central multipoles.

High resolution 1.3 μm overtone spectroscopy of HF dimer in a slit jet: K _{ a }=0←0 and K _{ a }=1←0 subbands of v _{acc}=2←0
View Description Hide DescriptionContinuous wave difference frequency mixing of a single mode Nd:YAG laser at 1.06 μm and a scanning, single mode ring dye laser (R6G) in a LiNbO_{3} crystal generates a novel source of widely tunable near infrared radiation in the 1.2–2.2 μm region. In conjunction with the high sensitivity of a pulsed slit nozzle expansion with multipass optics (0.48 m path length), this narrow band source of tunable ir light allows the high resolution study of overtone (v=2←0) spectra for a wide variety of molecular complexes with H stretching vibrations. In this paper, we report the first rotationally resolved spectra of (HF)_{2} in the first HF stretching overtone region. In particular, we observe K _{ a }=1←0 and 0←0 subbands for a vibrational state from one member of the v=2 overtone triad in (HF)_{2} with a band center of 7682.8228(5) cm^{−1}. We tentatively assign this state as the hydrogen bond acceptor (i.e., free) HF stretching overtone 2ν_{acc} based on predissociation line widths and excellent agreement with predictions based on an anharmonic local mode description of (HF)_{2}. Splittings of 0.2119(5) cm^{−1} (K ^{’} _{ a } = 0) and 0.0942(3) cm^{−1} (K ^{’} _{ a } = 1) due to interconversion tunneling are found.
From the observed intensity alternation due to nuclear spin statistical weights, the overall vibrational symmetry for K ^{’} _{ a } = 0 and 1 is unambiguously determined to be Γ_{vib}=A ^{+} and B ^{+} for the lower and upper tunneling levels, respectively. These A ^{+} and B ^{+} symmetry designations correspond to irreducible representations of the M_{ S4} molecular symmetry group, which allows for large amplitude motion and exchange of the identical HF subunits. Predissociation line broadening is observed in each of the four upper vibrational levels which varies between 56(20) and 175(25) MHz and depends sensitively on both K ^{’} _{ a } and the tunneling symmetry. This tunneling symmetry dependence, together with the unusual K ^{’} _{ a } dependence of the tunneling splitting and the anomalously large intensity ratio between the parallel and perpendicular transitions, indicates the presence of vibrational resonances in the overtone region not clearly evidenced in the analysis of the corresponding fundamental HF stretch region. Our results are discussed in the context of earlier static cell FTIR spectra and recent ab initio predictions for this overtone state. The data suggest that the overtone dynamics in (HF)_{2} can not be satisfactorily described as an oscillator pair connected by a 1D interconversion pathway, and may instead involve substantial coupling to other intermolecular vibrational degrees of freedom.

Departures from the soft collision model for Dicke narrowing: Raman measurements in the Q branch of D_{2}
View Description Hide DescriptionWith high quality spectral data, we have observed departures from the soft collision model for translational motion, in the transition region from Doppler broadening to Dicke narrowing. The departures are in agreement with theoretical calculations based on the Boltzmann equation. The implications of the results concerning the dynamics of fluids are discussed. In addition we show that the mass diffusion constant describes the translational diffusion of the optical coherence and we give precise measurements of the broadening coefficients of the Q(0) to Q(6) lines.

Theoretical and neon matrix electron spin resonance studies of the methanol cation: CH_{3}OH^{+}, CH_{3}OD^{+}, CH_{2}DOH^{+}, and ^{13}CH_{3}OH^{+}
View Description Hide DescriptionFour isotopes of the methanol cation radical (CH_{3}OH^{+}) have been generated by three independent methods and isolated in neon matrices at 4 K for a detailed electron spin resonance(ESR) investigation. The ion generation methods employed were Xirradiation,photoionization, and electron ionization. The nuclear hyperfine (Atensors) measurements were compared with those obtained from ab initio extended basis set multireference configuration interaction (CI) wave functions. The relationships between geometry and electronic structures were fully explored. The trend in the large isotropic methyl hydrogen A values for the isoelectronic series CH_{3}F^{+}, CH_{3}OH^{+}, and CH_{3}NH_{2} ^{+} was found to follow the trend in dissociation energies for these radical cations. The neon magnetic parameters for CH_{3}OH^{+} are g _{ x }=2.0036(4) and g _{ z }=2.010(1); A _{iso} (methyl hydrogens)=229(1) MHz, ‖A _{ x }‖=54(2) and ‖A _{ z }‖=80(3) MHz for the hydroxy hydrogen; ‖A _{ x }‖=40(2) and ‖A _{ z }‖=29(4) MHz for ^{13}C. The observed magnetic parameters for CH_{2}DOH^{+} indicate an unusually large deuterium effect A _{iso} (CH_{2})=329 MHz with ‖A _{iso}‖=4.1(3) MHz for the methyl deuterium. These results show that averaging of the methyl hydrogen environments is occurring on the ESR time scale.

Characterization of excited electronic states of naphthalene by resonance Raman and hyper‐Raman scattering
View Description Hide DescriptionThe first resonance Raman and hyper‐Raman scattering from naphthalene are reported. Fourth harmonic of a mode‐locked Nd:YAG laser is used to resonantly excite the ^{1} B _{1u } ^{+} transition, producing Raman spectra that confirm the dominance of the vibronically active ν_{28} (b _{3g }) mode and the Franck–Condon active a _{ g } modes, ν_{5} and ν_{3}. A synchronously pumped stilbene dye laser and its second harmonic are employed as the excitation sources for hyper‐Raman and Raman scattering from the overlapping ^{1} B _{2u } ^{+} and ^{1} A _{ g } ^{−} states. The Raman spectra indicate that the equilibrium geometry of naphthalene is distorted primarily along ν_{5}, ν_{8}, and ν_{7} normal coordinates upon excitation to ^{1} B _{2u } ^{+}. The hyper‐Raman spectrum shows that ν_{25} (b _{2u }) is the mode principally responsible for vibronic coupling between the ^{1} A _{ g } ^{−} and ^{1} B _{2u } ^{+} states. The results demonstrate the advantageous features of resonance hyper‐Raman scattering for the case of overlapping one‐ and two‐photon allowed transitions. Calculations based on simple molecular orbital configurations are shown to qualitatively agree with the experimental results.

High resolution photoelectron spectroscopy: The vibrational spectrum of the 2‐aminopyridine cation
View Description Hide DescriptionWe describe a new time‐of‐flight photoelectron spectrometer that combines molecular beam techniques with two‐photon ionization by a high repetition rate laser. The instrument routinely achieves a resolution of 5 meV. In a first application we studied the vibrational spectrum of the 2‐aminopyridine ion. By tuning the laser to various vibrational states of the intermediate S _{1}resonance we were able to assign the vibrational frequencies of modes 6a, 12, I ^{2} and 1. Other vibrational lines could be assigned by comparison with similar molecules. The ionization potential was found to be 8.099±0.003 eV, which differs from the literature value by 0.124 eV. The discrepancy might be explained by a fast intersystem crossing to a nearby triplet state.

Infrared spectroscopy of CO_{2}–D(H)Br: Molecular structure and its reliability
View Description Hide DescriptionA high resolution rovibrational absorptionspectrum of the weakly bonded CO_{2}–DBr complex has been recorded in the 2350 cm^{−1} region by exciting the CO_{2} asymmetric stretch vibration with a tunable diode laser. The CO_{2}–DBr band origin associated with this mode is 2348.2710 cm^{−1}, red‐shifted by 0.87 cm^{−1} from uncomplexed CO_{2}. The position of the hydrogen atom is determined from differences in moments‐of‐inertia between CO_{2}–DBr and CO_{2}–HBr, i.e., by using the Kraitchman method. From this, we conclude that ground state CO_{2}–H(D)Br has an average geometry that is planar and inertially T‐shaped, with essentially parallel HBr and CO_{2} axes. Average values of intermolecular parameters are: R _{cm}=3.58 Å, θ_{BrCO}=79.8°, and θ_{HBrC}=93.1°. The validity of using the Kraitchman method, which was designed for use with rigid molecules, with a floppy complex like CO_{2}–HBr is discussed. The experimental structure is corroborated qualitatively by results from Mo/ller–Plesset second‐order perturbation calculations, corrected for basis set superposition errors. The theoretical equilibrium geometry for the inertially T‐shaped complex is planar with structural parameters: R _{CBr}=3.62 Å, θ_{BrCO}=89°, and θ_{HBrC}=86°. A number of cuts on the four dimensional intermolecular potential surface confirm large zero‐point amplitudes, which are known to be characteristic of such systems, and these cuts are used to estimate tunneling splittings. Tunneling is shown to occur by out‐of‐plane rotation of the H atom, in accord with the experimental observations of Rice et al. There is no significant in‐plane tunneling. A quasilinear hingelike isomer (OCO–HBr) with R _{OH}=2.35 Å at equilibrium is calculated to be as stable as the T‐shaped complex; however, this species has yet to be observed experimentally. Photoinitiated reactions in CO_{2}–HX complexes are discussed.

Pressure tuning of spectral holes in organic crystalline materials: Irreversible effects
View Description Hide DescriptionThe behavior of spectral holes under pressure in a polycrystallinematerial, namely dimethyl‐s‐tetrazine (DMST) dopedn‐octane was investigated and compared with the behavior in a durene single crystal host and glasses. Application of pressure induces frequency shifts and line broadenings which are significantly larger than in single crystals and glasses. Part of the broadening is irreversible and is attributed to the creation of dipolar strain fields. The distribution of dislocation thresholds is continuous with no obvious lower cut off. The response of the material to pressure changes depends on its history.

Methyl rotor effects on acetone Rydberg spectra. I. The ^{1} A _{2}(3p←n)←^{1} A _{1} transition
View Description Hide DescriptionTwo‐photon spectra of the acetone ^{1} A _{2}(3p _{ x }←n)←^{1} A _{1}Rydberg transition obtained by injection seeded excitation determines a _{2}(ν_{12}) and b _{1}(ν_{17}) acetone ^{1} A _{2}Rydberg state torsional fundamental frequencies as 70.8 cm^{−1} (ν^{’} _{12}) and 129.4 cm^{−1} (ν^{’} _{17}) (compared to the ground‐state values 77.8 and 124.5 cm^{−1}, respectively). Corresponding values in (CD_{3})_{2}CO are 49.1 and 98.2 cm^{−1}, compared to 53.4 and 96.0 cm^{−1} in the ground state. The ^{1} A _{2} state 2ν^{’} _{12}, 3ν^{’} _{12}, and ν^{’} _{12} + ν^{’} _{17} energies are also assigned allowing determination of excited‐state potential constants. There are large changes (65–200 cm^{−1}) in these constants from those for the ground state causing significant perturbation in the shape of the ^{1} A _{2} state potential from that of the ground state. The ^{1} A _{2} constants are V _{3}=551, V _{33}=349, V ^{’} _{33}=−221, and V _{6}=0 cm^{−1}. The effective potential barrier height to internal rotation (202 cm^{−1}) is decreased from the ground state by about 15%, i.e., by ≊35 cm^{−1}; but the barrier height for eclipsed–eclipsed→staggered–staggered synchronous‐rotation is increased by ≊300 cm^{−1} (i.e., ≊35%) to 1103 cm^{−1}.

The Cu+F_{2} chemiluminescent reaction revisited. II. Kinetic studies
View Description Hide DescriptionOptical pumping, with a copper vapor laser, has been used to state select ground ^{2} S _{1/2} and metastable ^{2} D _{5/2} and ^{2} D _{3/2}copper atoms in a fast flow, low pressure reactor combined with a hollow cathode sputtering source. The absolute densities of these atoms (around 2×10^{11} and 10^{9} atoms cm^{−3} for ^{2} S and ^{2} D, respectively) were measured by resonant absorption and laser induced fluorescence techniques. Their isolated reactions with F_{2} have been studied in detail at 300 K. The total reaction cross section for Cu(^{2} S) is almost four times larger than for Cu*(^{2} D) metastable atoms. The strong chemiluminescent emission from electronically excited CuF* formed is attributed to the reaction of metastable Cu*(^{2} D) atoms, the Cu(^{2} S) reaction leading directly to ground state CuF(X ^{1}Σ^{+}) molecules. Spectral analysis of the chemiluminescence shows a highly inverted vibrational distribution in all energetically accessible excited states: a ^{3}Σ^{+}, b ^{3}Π, B ^{1}Σ^{+}, C ^{1}Π, and D ^{3}Δ_{1} of CuF with <f _{ v }≳≊0.7. Branching ratios for formation of the molecules in these states have been determined for reaction of copper atoms in each of the three electronic states. From the highest vibrational levels populated by reaction of Cu*(^{2} D _{5/2}) with F_{2}, v _{max}=21 and 26 for C ^{1}Π and b ^{3}Π states, respectively, one can deduce a more precise value of 33 560±240 cm^{−1} for the dissociation energy of the ground state CuF(X ^{1}Σ^{+}). The strong propensity observed for formation of CuF*(a,b,B,C,D) by Cu*(^{2} D,3d ^{9}4s ^{2})+F_{2} and CuF(X ^{1}Σ^{+}) by Cu(^{2} S,3d ^{10}4s)+F_{2}reactions can be explained by conservation of the copper atom ionic core throughout the course of the reaction.

The applicability of binary collision theories to complex molecules in simple liquids
View Description Hide DescriptionThe relaxation dynamics of vibrationally excited ground‐state azulene molecules have been examined by picosecond transient absorption spectroscopy in a variety of different solvents including hexane, chloromethanes, methanol, CClF_{3}, Xe, and Kr. A high pressure optical cell was used to liquify gases for use as solvents and to change their density and temperature independently over the entire liquid density range. Experimental results indicate that the vibrational cooling rate is strongly solvent dependent, with cooling rates of approximately 20 ps in molecular solvents and approximately 150 ps in atomic solvents. Comparison of the rates in Xe and Kr at constant number density demonstrates the strong effect of solvent mass on energy transfer. The effect of solvent temperature on vibrational cooling is minimal, as is the effect of solvent density. The latter result is quite surprising in light of earlier experiments on simpler molecular systems, such as I_{2} in Xe, and predictions of isolated binary collision theories. An explanation is offered from large scale molecular dynamics simulations of the system. In effect, azulene forms an ‘‘ordered Xe cluster’’ with xenon atoms; the xenon number density normal to the azulene molecular plane is independent of solvent density.

Collisional relaxation of vibrational excitation: Effects of bath gas structure
View Description Hide DescriptionInfrared multiple photondissociation has been used to study the relaxation of vibrationally excited trifluoroacetate anion. The internal energy of the ensemble of ions was monitored by measuring the extent of photodissociation in a Fourier‐transform, ion cyclotron resonancespectrometer. Bimolecular quenching rate constants are measured in the presence of excess bath gases. Comparison of the experimental collision efficiencies with calculations of purely statistical energy redistribution has been done and indicates that species which can interact chemically with a trifluoroacetate anion exhibit a marked increase in the efficiency of collisional deactivation. Systematic use of the ratio R, defined as the average energy transferred per collision derived from experiment divided by the average energy transferred calculated from a statistical model, is proposed. A correlation of R with the hypothetical, limiting statistical lifetimes of the collision complexes is observed. This suggests that both the number and nature of oscillators of the bath gases as well as the intermolecular well depths with trifluoroacetate are important in determining the extent of energy transfer. It further suggests that energy transfer is limited by the collision duration.

Statistical modeling of ion–molecule electrostatic capture
View Description Hide DescriptionA statistical model is presented which enables rapid and accurate calculation of capture rate coefficients for the interaction of ions with neutrals possessing any combination of dipole, quadrupole, and isotropic or anisotropic induced‐dipole moments. Rate coefficients at very low temperatures are calculated via the state‐resolved statistical adiabatic channel model (SACM). At higher temperatures, a classical state‐counting technique is utilized which may be derived either from microcanonical variational transition state theory or from the high‐temperature limit of the quantized SACM approach. The predicted rate coefficients are generally accurate to within 10% in comparison with classical trajectory calculations.

Evaluation of thermal rate constants in the eigenbasis of a Hamiltonian with an optical potential
View Description Hide DescriptionMiller and co‐workers [J. Chem. Phys. 61, 1823 (1974); ibid., 79, 4889 (1983)] have derived an exact quantum mechanical expression for reactive thermal rate constants in terms of the time integral of a flux autocorrelation function. The evaluation of this integral in a finite basis poses the problem that spurious oscillations in the correlation function due to recurrences can occur at long times, corrupting the result. To obviate this difficulty, we add to the Hamiltonian an optical potential in the asymptotic region, and evaluate eigenvalues and eigenvectors using the technique of successive truncation. These operations allow a diagonal (although nonorthogonal) representation of the propagator in which the eigenvalues are exponentially decaying functions of time, which damp the components of the propagated vectors before the spurious reflection back into the interaction region. In this manner, the infinite time limit of the integral may be evaluated properly. Furthermore, the results of a single diagonalization may be used to compute the thermal rate constant over a range of temperatures.

Temperature‐jump measurements on the kinetics of association and dissociation in weakly bound systems: N_{2}O_{4}+M=NO_{2}+NO_{2}+M
View Description Hide DescriptionKinetic measurements have been made on the gas‐phase reaction N_{2}O_{4}+M=2NO_{2}+M by applying the temperature‐jump relaxation method. Equilibrium mixtures containing nitrogen dioxide, dinitrogen tetroxide, a small quantity of an IR absorber, and an excess of a third‐body bath gas, have been subjected to a short CO_{2} laser pulse. The induced temperature jump, of the order of 1 K, displaces the equilibrium towards NO_{2} formation. After the fast heating of the mixture, the time dependence of the NO_{2} and N_{2}O_{4} concentrations has been monitored at 420 and 250 nm, respectively. For small perturbations, the relaxation to the new equilibrium concentrations at the higher temperature follows first‐order kinetics, and the thermal rate constant can be deduced from the measured relaxation rate constant.Measurements have been made at 255 K in the pressure range between 0.3 and 200 bars He, which corresponds to the falloff range for this reaction. The high‐ and low‐pressure limiting rate constants,k _{rec,∞}=(7.0±0.7)×10^{11} cm^{3} mol^{−1} s^{−1} and k _{rec,0}/[He]=(2.1±0.2)×10^{14} cm^{3} mol^{−1} s^{−1}, respectively, extrapolated from the experimental data at 255 K, are in agreement with those calculated with a simplified statistic adiabatic channel model. The temperature dependence of the high‐ and low‐pressure recombination rate constant, determined between 255 and 273 K, is given by the relations k _{rec,∞}=(2.2±0.2)×10^{6}×T ^{(2.3±0.2)} cm^{3} mol^{−1} s^{−1} and k _{rec,0}/[He]=(7.5±0.8)×10^{35}×T ^{(−9.0±0.9)} cm^{3} mol^{−1} s^{−1}. The corresponding high‐ and low‐pressure dissociationrate constants are k _{diss,∞}= (2.8±0.3)×10^{13}×T ^{(1.3±0.2)} exp{−(6790±700)/T} s^{−1} and k _{diss,0}/[He]=(9.6±0.9)×10^{42} T ^{(−10.0±1.0)} ×exp{−(6790±700)/T} s^{−1}, respectively.