Index of content:
Volume 99, Issue 6, 15 September 1993

Silyl group internal rotation in S _{1} phenylsilane and phenylsilane cation: Experiments and ab initio calculations
View Description Hide DescriptionResonant two‐photon ionization (R2PI) and pulsed field ionization (PFI) were used to measureS _{1}–S _{0} and cation–S _{1} spectra of internally cold phenylsilane. We measure the adiabatic ionization potentials IP(phenylsilane)=73 680±5 cm^{−1}, IP(phenylsilane ⋅Ar)=73 517±5 cm^{−1} and IP(phenylsilane ⋅Ar_{2})=73 359±5 cm^{−1}. We assign many low lying torsion–vibration levels of the S _{1} (Ã ^{1} A _{1}) state and of X̃ ^{2} B _{1} of phenylsilane^{+}. In both states, the pure torsional transitions are well fit by a simple sixfold hindered rotor Hamiltonian. The results for the rotor inertial constant B and internal rotation potential barrier V _{6} are, in S _{1}, B=2.7±0.2 cm^{−1} and V _{6}=−44±4 cm^{−1}; in the cation, B=2.7±0.2 cm^{−1} and V _{6}=+19±3 cm^{−1}. The sign of V _{6} and the conformation of minimum energy are inferred from spectral intensities of bands terminating on the 3a ^{‘} _{1} and 3a ^{‘} _{2} torsional levels. In S _{1} the staggered conformation is most stable, while in the cation ground state the eclipsed conformation is most stable. For all sixfold potentials whose absolute phase is known experimentally, the most stable conformer is staggered in the neutral states (S _{0} and S _{1} p‐fluorotoluene, S _{1} toluene, S _{1} p‐fluorotoluene) and eclipsed in the cationic states (ground state toluene^{+} and phenylsilane^{+}). In phenylsilane^{+} we estimate several potential energy coupling matrix elements between torsional and vibrational states.
For small V _{6}, the term P _{α} P _{ a } in the rigid‐frame model Hamiltonian strongly mixes the 6a ^{’} _{1} and 6a ^{’} _{2} torsional states, which mediates further torsion–vibrational coupling. In addition, the cation X̃ ^{2} B _{1} vibrational structure is badly perturbed, apparently by strong vibronic coupling with the low‐lying Ã ^{2} A _{2} state. Accordingly, ab initio calculations find a substantial in‐plane distortion of the equilibrium geometry of the X̃ ^{2} B _{1} state, while the Ã ^{2} A _{2} state is planar and symmetric. The calculations also correctly predict the lowest energy conformer for S _{0} states and for cation ground states. Finally, we adapt the natural resonance theory (NRT) of Glendening and Weinhold to suggest why sixfold barriers for methyl and silyl rotors are uniformly small, while some threefold barriers are quite large. The phase of the sixfold potential is apparently determined by a subtle competition between two types of rotor‐ring potential terms: attractive donor–acceptor interactions and repulsive van der Waals interactions (steric effects).

The accurate determination of spectroscopic constants for a dark vibrational state: The Coriolis coupled ν_{4} and ν_{6} bands of D^{11}BF_{2}
View Description Hide DescriptionThe high resolution infrared spectrum of deuterated difluoroborane, DBF_{2}, has been recorded from 700 to 912 cm^{−1}. At low resolution, the only observable bands in this region belong to the c‐type, out‐of‐plane deuterium motion, ν_{4}, of the two B isotopomers. Upon analysis of the ν_{4} fundamental in D^{11}BF_{2}, a large perturbation was observed due to a ΔK=±1 Coriolis interaction with a dark vibrational state. The dark state was presumed to be the unseen, a‐type, in‐plane D–B rocking fundamental, ν_{6}. A novel method of upper state energy level interlacing was used to determine A, B+C and T _{0} for the dark excited state, 6^{1}, the band origin, rotational and centrifugal distortion constants for the 4^{1} state, and ξ_{4,6} ^{ b } (≊2ζ_{4,6} ^{ b } B). Based upon these constants, ν_{6} transitions were predicted and located in the region of the perturbation. Experimentally determined Coriolis, rotational, and centrifugal distortion constants are reported. The experimental results are compared with the results from ab initio calculations.

Control of molecular vibrational excitation and dissociation by chirped intense infrared laser pulses. Rotational effects
View Description Hide DescriptionWe extend our previous studies on control of dissociation and vibrational excitation of a diatomic molecule using chirped, intense, infrared laser pulses [Phys. Rev. Lett. 65, 2355 (1990)]. The present model includes molecular rotations and a realistic molecular dipole function. The results obtained from numerical integration of the time‐dependent Schrödinger equation show a considerable sensitivity of dissociation probabilities to the initial rotational quantum number. Although rotational effects generally decrease the excitation efficiency compared to previous nonrotating molecule results, the dissociation probability induced by chirped pulses is still four to eight orders of magnitudes greater than that for monochromatic pulse dissociation.

Hyperfine interactions in the ground states of titanium monoxide and mononitride
View Description Hide DescriptionA comparative study of the hyperfine interactions in the X ^{2}Σ^{+} state of TiN and the X ^{3}Δ state of TiO has been performed. The ^{48}Ti^{14}N(I=1) hyperfine structure was determined from the analysis of 19 components of the N=1–0 and N=2–1 pure rotational transitions recorded using the pump/probe microwave‐optical double resonance technique. The ^{47}Ti(I=5/2) hyperfine structure of X ^{2}Σ^{+} TiN was determined from an analysis of the high resolution optical spectrum of the (0,0) A ^{2}Π_{3/2}–X ^{2}Σ^{+} band system. The resulting parameters are (in MHz) B(^{48}Ti^{14}N)=18 589.3513(13), D(^{48}Ti^{14}N)=0.026 31(18), γ(^{48}Ti^{14}N)=−52.2070(13), b _{ F }(N)=18.480(3), c(N)=0.166(7), eQq _{0}(N)=−1.514(8), C _{ I }(N)=0.0137(12), b _{ F }(^{47}Ti) =−558.8(11), c(^{47}Ti)=−15(5), and eQq _{0}(^{47}Ti)=62(16). An analysis of the (0,0) band of the B ^{3}Π–X ^{3}Δ system of ^{47}Ti^{16}O produced the X ^{3}Δ hyperfine parameters (in MHz): a(^{47}Ti) =−54.7(21), (b _{ F }+2c/3)(^{47}Ti)=−231.6(60), and eQq _{0}(^{47}Ti)=−49(31). An interpretation based upon the predicted nature of the bonding in TiO and TiN is given.

Spectroscopy of the indium argon van der Waals complex: A high resolution study of the B ^{2}Σ_{1/2}←X2 ^{2}Π_{3/2} system
View Description Hide DescriptionThe InAr van der Waals complex has been characterized by high resolution laser induced fluorescence excitation spectroscopy. Six vibronic bands of the B ^{2}Σ_{1/2}←X2 ^{2}Π_{3/2} transition have been observed and five of these (v’,0), where v’=1–5, have been rotationally analyzed. Rydberg–Klein–Rees potential curves were constructed for the B ^{2}Σ_{1/2} state using the rotational and vibrational constants determined from these spectra. Equilibrium bond lengths were determined for the B and X2 states and a dissociation energy was determined for the B state. The stronger bonding present in the B state is rationalized in terms of penetration of the argon atom into the diffuse 6s orbital of indium. Evidence is presented that the B state potential energy curve has a barrier at long range, due to Pauli repulsion, of ∼60 cm^{−1}. An analysis of the hyperfine structure involving the ^{115}In nucleus was made. It is concluded that the X2 state conforms to Hund’s coupling case a _{β}, whereas the B state conforms to case b _{βs }. The extent of 6s–6p hybridization in the upper state was measured from hyperfine splittings and was used in conjunction with a simple electrostatic model to estimate the polarizability of the indium atom in the 6s ^{2} S _{1/2} state. A value of 68(4) Å^{3} was obtained (1σ error).

Difference‐frequency laser spectroscopy of the 3ν_{2}+ν_{3} band of C_{2}H
View Description Hide DescriptionA spectrum of C_{2}H ^{2}Π–^{2}Σ^{+} transition centered at 2928 cm^{−1} has been measured in the gas phase using a high resolution difference‐frequency infrared laserspectrometer. The C_{2}H molecules are produced in a hollow cathode discharge through a mixture of C_{2}H_{2} (∼20 mTorr) and H_{2} (∼500 mTorr). The discharge amplitude modulation technique is used to enhance sensitivity and selectivity. About 125 lines are rotationally analyzed and accurate molecular constants are determined through a least‐squares fit. The band is assigned to the 3ν_{2}+ν_{3} combination band. The assignment is consistent with the observed ^{2}Π–^{2}Σ^{+} symmetry and is in good agreement with the recent theoretical calculation of the vibrational frequency and the spin–orbit coupling constant. The relatively large spin–orbit coupling constant of the (0 3 1) state is indicative of strong vibronic interaction between the Ã and X̃ states. A small perturbation appears near the end of observed Q‐branch series, which is probably caused by a Σ vibronic state. New assignments of the progressions (0 v _{2} 0) and (0 v _{2} 1) toward higher v _{2} are proposed based on the existing gas phase and rare gas matrix data with the help of recent ab initiotheoretical predictions.

On the vibronic structure of the S _{0}↔S _{1} transitions in azulene
View Description Hide DescriptionThe semiempirical (quantum chemical consistent force field/π‐electron and complete neglect of differential overlap/spectroscopic parametrization) and ab initio methods are used to study the vibronic structure of the optical transitions between the S _{0} and S _{1} states of azulene. It is shown that although the excited state is described quite well as arising from the promotion of one electron from the highest occupied molecular orbital to the lowest unoccupied one, the inclusion of doubly excited configurations in description of the wave functions of the two states in question is essential for a proper characterization of the geometry change upon electronic excitation and, therefore, for the resulting Franck–Condon structure of the absorption and emission spectra. The vibronically induced intensities of the b _{2} fundamentals in the two spectra are calculated and compared with the experimental data, and the problem of correlation between the b _{2} modes in the two states is solved.

The electric dipole moment of NO A ^{2}Σ^{+} v’=0 measured using Stark quantum‐beat spectroscopy
View Description Hide DescriptionWe report measurements and analysis of Stark quantum beats observed in the fluorescence of nitric oxide (NO) from which we determine the electric dipole moment of the A ^{2}Σ^{+} v’=0 state. A pulse‐amplified cw dye laser was used to excite the A–X (0,0) Q _{1}(1) transition of ^{14}N^{16}O in electric fields up to 22.5 kV/cm. Fourier analysis of the time‐resolved laser‐induced fluorescence signals yielded Stark tunings for each of the six ‖M _{ F }‖ hyperfine sublevels in the N=1, J=3/2 spin‐rotational level. The measurements were fit to a model Hamiltonian including fine, hyperfine, and Stark matrix elements. The resulting dipole moment was then corrected for polarizability effects to yield a value for the A ^{2}Σ^{+} v’=0 state of μ_{ A }=1.08±0.04 D. This result compares favorably to a previous measurement of μ_{ A } in v’=3 and to our quantum theoretical calculations of the A ^{2}Σ^{+} v’=0 state reported here.

Photoelectron angular distributions and vibrational branching ratios of CO (2+1)‐photon ionization via the B ^{1}Σ^{+} state
View Description Hide DescriptionWe present vibrational branching ratios and photoelectron angular distributions for resonantly enhanced (2+1)‐photon ionization of CO. The excitation ladder involves the B ^{1}Σ^{+}(v _{ i }=0 or 1) Rydberg state. Contrary to expectations fostered by the Franck–Condon principle, ionization via v _{ i }=0 branches into vibrational statesv ^{+}=0–4. Such phenomena are also observed in the case of v _{ i }=1, but only to a minor extent. The angular emission patterns of the photoelectrons ejected during the ionizing step are of distinct character in that they are highly anisotropic for Δv=v _{ i }−v ^{+}=0 processes, but show isotropy when due to Δv≠0 transitions. The photoelectron angular distribution which accompanies the Δv=0 ionization of B(v _{ i }=1) shows p‐wave character, and hence we may postulate a spherical potential of the Rydberg ion core. The same approximation should hold for the v _{ i }=0 state. The dissimilar appearance of the angular distributions when ionizing from this level invites the hypothesis of two individually different ionization channels.

Microwave spectrum, quadrupole coupling constants, structure and ab initio calculations of N‐bromocyanofluoromethanimine
View Description Hide DescriptionThe microwave spectrum of N‐bromocyanofluoromethanimine CF(CN)NBr in the ground vibrational state in the region from 18.0 to 38.0 GHz has been investigated. The assigned spectrum is only consistent with a planar molecule with the cyano group trans to the bromine atom. The hyperfine structure due to the nuclear quadrupole coupling of the bromine nucleus has been analyzed for the a‐type R transitions of both the ^{81}Br and ^{79}Br isotopic species. Quadrupole splittings due to the ^{14}N nuclei were not resolved. The rotational constants, the centrifugal distortion constant Δ_{ J }, and the nonzero values of the diagonal and off‐diagonal elements of the bromine quadrupole coupling tensor were determined by a least‐squares fit for both isotopic species of bromine. The r _{0} structural parameters r(C–C), r(C–F), and ∢(N=C–C) were determined to have values of 1.422(9) and 1.349(15) Å, and 121.6(9)°, respectively, whereas other structural parameters were fixed at the values obtained for the corresponding parameters of similar molecules. The equilibrium geometry of the two possible isomeric forms of CF(CN)NBr were determined by an ab initio calculation which employed the various basis sets with and without electron correlations. These results are compared with the corresponding values of some similar molecules.

High resolution, jet‐cooled infrared spectroscopy of (HCl)_{2}: Analysis of ν_{1} and ν_{2} HCl stretching fundamentals, interconversion tunneling, and mode‐specific predissociation lifetimes
View Description Hide DescriptionAn extensive series of near‐infrared absorption spectra are recorded for jet‐cooled (6–14 K) hydrogen chloride dimer (HCl)_{2}. Both ΔK _{ a }=0 and ΔK _{ a }=±1 bands are observed for both the free (ν_{1}) and bonded (ν_{2}) HCl stretches; all three chlorine isotopomers (H ^{35}Cl–H ^{35}Cl, H ^{35}Cl–H ^{37}Cl, and H ^{37}Cl–H ^{37}Cl) are observed and analyzed for K ^{‘} _{ a } ≤ 2. The slit jet spectrum extends significantly the previous cooled cell infrared study of this complex and provides a measure of tunneling splittings for K _{ a }=0 and 1 for each of the HCl ground (v=0) and excited (v=1) states. Mode specific vibrational predissociation is observed via analysis of the absorption line shapes, with Lorentzian contributions to the line profiles of Δν_{1}≲1.6 MHz and Δν_{2}=5.1±1.2 (2σ) MHz full width at half‐maximum for ν_{1} and ν_{2} excitation, respectively. Stronger coupling in (HCl)_{2} of the bonded (ν_{2}) vs free (ν_{1}) HCl vibration to the dissociation coordinate is consistent with the comparable trends observed in other hydrogen bonded dimers. Quantum mechanical variational calculations on an electrostatic angular potential energy surface are used to model the internal HCl rotor dynamics using a coupled rotor formalism; analysis of the internal rotor eigenfunctions provides direct evidence for large amplitude ‘‘geared’’ internal rotation of the HCl subunits.

Molecular dynamics and semiclassical electronic spectra of naphthalene⋅Ar_{ n } clusters (n≤4)
View Description Hide DescriptionMolecular dynamics simulations were performed for van der Waals clusters naphthalene⋅Ar_{ n }, n=1 to 4. For all isomers and conformers of these clusters, dynamical quantities such as velocity autocorrelation functions, vibrational power spectra, and semiclassical electronic absorption spectra were calculated over a wide energy range, and averaged over a canonical distribution at temperatures in the range T=5 to 30 K. Electronic absorption spectra were calculated for the origin bands according to the semiclassical method [L. E. Fried and S. Mukamel, J. Chem. Phys. 96, 116 (1992)] and are compared with the corresponding experimental naphthalene⋅Ar_{ n } R2PI spectra [T. Troxler and S. Leutwyler, J. Chem. Phys. 95, 4010 (1991)]. The appearance of distinct absorption bands due to specific isomers for a given cluster size, as observed experimentally, is well reproduced by the simulations. Comparison of calculated electronic shifts for different isomers allows clear assignments in the experimental spectra. Increasing the simulation temperature to T=15–25 K is accompanied by band broadening and the appearance of sidebands towards the blue. Especially strong sidebands appear for naph⋅Ar_{2} and all clusters containing the Ar_{2} subunit, due to large‐amplitude surface rotation/translation of the argon dimer on the naphthalene surface, in agreement with experiment. For clusters containing the n=3 and n=4 subunits the spectral broadening is smaller. For the n=4 (4+0)‐isomer, the calculated band shape increases less than for the other n=4 isomers, mainly due to the motional narrowing effect of cluster fluxionality. Above 25 K, isomerization between different possible topological structures also occurs by side‐crossing motion of one or several argon atoms.

Vibration–rotation variational calculations: Precise results on HCN up to 25 000 cm^{−1}
View Description Hide DescriptionVariation calculations of the vibration–rotation energy levels of many isotopomers of HCN are reported, for J=0, 1, and 2, extending up to approximately 8 quanta of each of the stretching vibrations and 14 quanta of the bending mode. The force field, which is represented as a polynomial expansion in Morse coordinates for the bond stretches and even powers of the angle bend, has been refined by least squares to fit simultaneously all observed data on the Σ and Π state vibrational energies, and the Σ state rotational constants, for both HCN and DCN. The observed vibrational energies are fitted to roughly ±0.5 cm^{−1}, and the rotational constants to roughly ±0.0001 cm^{−1}. The force field has been used to predict the vibration rotation spectra of many isotopomers of HCN up to 25 000 cm^{−1}. The results are consistent with the axis‐switching assignments of some weak overtone bands reported recently by Jonas, Yang, and Wodtke, and they also fit and provide the assignment for recent observations by Romanini and Lehmann of very weak absorption bands above 20 000 cm^{−1}.

A molecular theory of the line shape: Inhomogeneous and homogeneous electronic spectra of dilute chromophores in nonpolar fluids
View Description Hide DescriptionKubo’s stochastic theory of the spectralline shape provides an elegant phenomenological description of inhomogeneous and homogeneous broadening and the transition between the two. This theory has been used profitably in the analysis of many experiments. In this paper we attempt to provide a microscopic foundation for the Kubo model by developing a completely molecular theory of the line shape. For definiteness we focus on the optical line shape of dilute chromophores in nonpolar fluids. Many of the features of the Kubo theory are found in the molecular theory; indeed, the molecular theory produces microscopic expressions involving the solvent structure and dynamics for Kubo’s phenomenological parameters, and provides some justification for the Gaussian assumption in the stochastic theory. On the other hand, the molecular theory produces a transition frequency time‐correlation function that is distinctly nonexponential, in contrast to the exponential assumption of the Kubo theory, and it is found that this nonexponentiality is necessary for the accurate description of line shapes in the regime intermediate between inhomogeneous and homogeneous broadening. For a model of Lennard‐Jones particles the molecular theory is compared with molecular dynamicscomputer simulations.

Kinetics for the quenching and relaxation of boron oxide
View Description Hide DescriptionLaser induced fluorescence studies are described in this paper for the kinetics of BO(A ^{2}Π) quenching, vibrational and rotational relaxation, and for the radiative lifetime of the A ^{2}Π state. Measured electronic quenching rates with several gases are presented. The rates are reasonably slow with He, Ar, N_{2}, N_{2}O, and CF_{4}, as the collisional partners. The gases O_{2}, F_{2}, NO, and NO_{2} show a very fast quenching of the A ^{2}Π state, probably as a consequence of chemical reaction with BO along with collisional energy transfer. Vibrational relaxation is generally slow compared to quenching except with CF_{4} where the collisional partner has a vibrational structure in near resonance with that of BO(A ^{2}Π). Rotational relaxation in BO(A) by He, NO, O_{2}, CF_{4}, and SF_{6} was found to be fast, as would be expected in a molecule with closely spaced energy levels. The effects on the BO spectra of vibrational transfer with CF_{4} are shown. Quenching rates for BO(A ^{2}Π) v’=0 and 2 measured over a series of helium pressure extrapolate to values of 1.75 and 1.37 μs for the radiative lifetimes of the v’=0 and 2 levels.

Magnetism and spin dynamics in MnPS_{3} and pyridine intercalated MnPS_{3}: An electron paramagnetic resonance study
View Description Hide DescriptionA detailed investigation of the magnetic properties of single crystals of the two‐dimensional layered Heisenberg antiferromagnet MnPS_{3} and pyridine intercalated MnPS_{3} by static magnetic susceptibility and electron paramagnetic resonance(EPR) is reported. A detailed analysis of the angular and temperature variation of the EPRlinewidths and line shapes has been carried out. While MnPS_{3} orders antiferromagnetically at T _{ N }=78 K, the pyridine intercalated compound exhibits isotropic weak ferromagnetism below 70 K. It is known that in the intercalated compound there is a loss of center of inversion so that a canting of spins could arise from either Dzyaloshinsky–Moriya (DM) d _{ ij } S _{ i }×S _{ j } interaction or single ion anisotropy,DS _{ z } ^{2}. The angular dependence of EPRlinewidth and line shape of MnPS_{3} even at 300 K shows none of the features expected for a low‐dimensional magnetic system at high temperatures; a (3 cos^{2} θ−1)^{2} angular dependence of linewidth and Lorentzian line shape at θ=55°. Instead, over the entire temperature range studied, the linewidth showed a 1+cos^{2} θ dependence and the line shapes predominantly Gaussian. The linewidth as well as the Gaussian character increased with decreasing temperature. The results have been interpreted to show that there is substantial intralayer spin–spin correlations and that critical fluctuations manifest at temperatures much above T _{ N }. The pyridine intercalated compound, however, shows the expected W‐shaped angular dependence of linewidth with a minimum at θ=55° and 135°. The angular variation of linewidth could be fitted to a A+B cos^{2} θ+C(3 cos^{2} θ−1)^{2} dependence. However, the near‐Lorentzian line shape at all angles implies that other relaxation mechanisms are operative which is also evidenced from the cos^{2} θ contribution to the angular variation of linewidth. Evidence for both DM as well as single ion anisotropy in the intercalated compound is presented. The presence of both of the above terms would explain the isotropic weak ferromagnetism observed in the static susceptibility of pyridine intercalated MnPS_{3}.

The near ultraviolet photodissociation dynamics of azomethane
View Description Hide DescriptionThe photodissociation of azomethane following absorption of a single 351 nm photon was studied using the method of molecular beam photofragment translational spectroscopy. The dissociation was observed to proceed via cleavage of both C–N bonds to yield N_{2} and two methyl radicals. The measured time‐of‐flight spectra show evidence that the two methyl radicals possess unequal velocities in the azomethane center of mass suggesting that the dissociation is not symmetric. The angles between the asymptotic center‐of‐mass velocities for all three fragments are strongly correlated, implying that the methyldiazenyl radical (CH_{3}N_{2}) intermediate decomposes within a fraction of its rotational period. We conclude, therefore, that the dissociation is concerted, not stepwise as was inferred from recent time‐resolved experiments. The overall translational energy distributions for all the photofragments in the azomethane center of mass reveal that an average of 60% of the total available energy appears as translation. A possible mechanism, consistent with the experimental findings, will be proposed and discussed.

Femtosecond real‐time probing of reactions. XI. The elementary OClO fragmentation
View Description Hide DescriptionFemtosecondreactiondynamics of OClO in a supersonic molecular beam are reported. The system is excited to the A ^{2} A _{2} state with a femtosecond pulse, covering a range of excitation in the symmetric stretch between v _{1}=17 to v _{1}=11(308–352 nm). A time‐delayed femtosecond probe pulse ionizes the OClO, and OClO^{+} is detected. This ion has not been observed in previous experiments because of its ultrafast fragmentation. Transients are reported for the mass of the parent OClO as well as the mass of the ClO. Apparent biexponential decays are observed and related to the fragmentation dynamics: OClO+hν→(OClO)^{‡}*→ClO+O →Cl+O_{2} . Clusters of OClO with water (OClO)_{ n } (H_{2}O)_{ m } with n from 1 to 3 and m from 0 to 3 are also observed. The dynamics of the fragmentation reveal the nuclear motions and the electronic coupling between surfaces. The time scale for bond breakage is in the range of 300–500 fs, depending on v _{1}; surface crossing to form new intermediates is a pathway for the two channels of fragmentation: ClO+O (primary) and Cl+O_{2} (minor). Comparisons with results of ab initio calculations are made.

Transition state structures and angular momentum effects in the dissociation dynamics of energy‐selected C_{4}H^{+} _{8} ions
View Description Hide DescriptionThe photoionization and dissociation dynamics of energy‐selected 1‐butene ions have been investigated by the technique of threshold photoelectron photoion coincidence (TPEPICO) time of flight mass spectrometry. The absolute dissociation rates for the reactions leading to the loss of H, CH_{3}, and CH_{4} have been measured for two samples prepared with very different internal energy and angular momentum distributions. First rotationally cold ions were prepared by photoionizing 1‐butene molecules cooled in a seeded molecular beam. These rates were analyzed within the framework of RRKM theory with vibrator transition state structure for all three channels. Excellent agreement between theory and experiment was obtained when ab initio calculated transition state frequencies were used for the H loss and the CH_{3} loss transition states. A variational transition state theory (VTST) analysis shows that the CH_{3} loss transition state lies about 11 kJ/mol below the dissociation limit. Second, dissociation rates using an effusive source which contained a 298 K distribution of vibrational and rotational energy were measured. The vibrator‐type transition state model, with inclusion only of the vibrational energy distribution, gives a good account of the total rates but significantly overestimates the H loss branching ratio. Excellent agreement is obtained, however, when the energies of the molecular ions and vibrator transition states are corrected for the rotational energy of each structure. K‐rotor mixing with the vibrations does not change the calculated rates significantly. Finally, the analysis confirms a previous proposal [Faraday Discuss. Chem. Soc. 75, 57 (1983)] that an orbiting transition state (a la phase space theory) is not the rate limiting bottleneck at the energies used in this experiment.

An investigation of the 355 nm photodissociation of NO_{2} by state‐resolved photofragment imaging
View Description Hide DescriptionThe 355 nm photodissociation of NO_{2} cooled in a supersonic beam has been investigated by state‐resolved photofragment imaging. The NO and O(^{3} P _{ J }) photofragments were state‐selectively ionized and projected onto a two‐dimensional, position‐sensitive detector to obtain speed and angular distributions. The speed distribution of the O(^{3} P _{2}) fragment displays two peaks corresponding to oxygen produced in coincidence with NO(υ=0) and NO(υ=1). The angular distributions for the O(^{3} P _{2}) and for the NO in several vibrational and rotational levels can be characterized by an anisotropy parameter of β=1.2±0.3. This value, while higher than that measured previously, is consistent with a dissociation lifetime on the order of 200–400 fs and with the colder rotational temperature of the current beam experiment. The rotational distributions of the NO product are found to be in good agreement with other recent measurements.