Recoil Anisotropy Parameter Research Articles

Probe of I( 2P j) atoms using two-photon resonant four-wave mixing spectroscopy following the 266-nm photodissociations of various alkyl and perfluoroalkyl iodides

Two-photon resonant four-wave mixing spectroscopy has been utilized successfully for probing I and I ∗ nascent from the 266 nm photodissociations of various alkyl and perfluoroalkyl iodides. The relative quantum yields for I ∗, as well as the recoil anisotropy parameters, were extracted. Recoil anisotropy parameters close to the limit value for parallel transition indicates that the ground-state I( 2P 3/2) originates from the curve crossing from the initially prepared 3Q 0 to 1Q 1 state. The curve-crossing probabilities for alkyl and perfluoroalkyl iodides were obtained and tried to explain using the well-known Landau–Zener model. In the cases of perfluoroalkyl iodides, it is necessary to consider the fluorination effect to describe the detailed dynamics observed in this study and has been revealed that the electronic effect due to fluorine atom substitution is dominant, leading to decrease in the curve-crossing probability.

Electronic Spectroscopy and Photodissociation Dynamics of the 1-Hydroxyethyl Radical CH3CHOH

The electronic spectroscopy and photodissociation dynamics of the CH3CHOH radical in the region 19,400-37,000 cm(-1) (515-270 nm) were studied in a molecular beam using resonance-enhanced multiphoton ionization (REMPI), photofragment yield spectroscopy, and time-of-flight (TOF) spectra of H and D fragments. The onset of the transition to the Rydberg 3s state, the lowest excited state, is estimated at 19,600 +/- 100 cm(-1). The 3s state dissociates fast, and no REMPI spectrum is observed. The origin band of the transition to the 3pz state, identified by 2 + 2 REMPI, lies at 32,360 +/- 70 cm(-1), and a vibrational progression in the C-O stretch is assigned. When exciting CH3CHOH near the onset of the unstructured absorption to the 3s state, only one peak is observed in the center-of-mass (c.m.) translational energy (Et) distribution obtained by monitoring H photofragments. The measured recoil anisotropy parameter beta = -0.7 +/- 0.1 is typical of a perpendicular transition. The O-H bond energy is determined to be D0 = 1.1 eV +/- 0.1 eV. At excitation energies >31,200 cm(-1) (3.87 eV) a second, low Et peak appears in the c.m. Et distribution with beta approximately 0. Its relative intensity increases with excitation energy, but its beta value does not change. In contrast, the beta value of the higher Et peak becomes monotonically less negative at higher excitation energies, decreasing to -0.2 +/- 0.1 at 35,460 cm(-1). By comparison of the TOF distributions of the isotopologs CH3CHOH, CH3CHOD, and CD3CHOH, it is concluded that two major product channels dominate the photodissociation, one leading to acetaldehyde and the other to vinyl alcohol (enol) products. There is no indication of isomerization to ethoxy. It appears that separate conical intersections lead to the observed channels, and the dynamics at the conical intersection and the exit channel deposit much of the available energy into internal energy of the products.

Nonadiabatic dynamics in the photodissociation of ICH2CN at 266 and 304nm studied by the velocity map imaging

Photodissociation dynamics of iodoacetonitrile (ICH2CN) have been investigated at pump wavelengths of 266 and 304 nm using a photofragment ion image velocity mapping technique. At both wavelengths, the prompt C-I bond rupture takes place on the repulsive excited states to give I(2P3/2) and I*(2P1/2), and their speed and spatial distributions are simultaneously measured. The recoil anisotropy parameter (beta) at 266 nm is determined to be 1.10 and 1.60 for I and I*, respectively, while it is found to be much higher at 304 nm to give beta=1.70 and 1.90 for I and I*, respectively. The branching ratios for I*I channels are measured to be 0.724 and 0.136 at 266 and 304 nm, respectively, giving insights on nonadiabatic transition phenomena and relative oscillator strengths of optically accessible transitions of ICH2CN. Accordingly, relative oscillator strengths of parallel/perpendicular transitions and nonadiabatic transitions among the excited states are quantitatively characterized. A large portion of the available energy (41%-48%) goes into the internal energy of the CH2CN fragment. A modified impulsive model in which the CH2CN fragment is assumed to be rigid predicts the energy disposal quite well. Delocalization of an unpaired electron of the CH2CN radical during the C-I bond cleavage, leading to a large structural change of the CH2CN moiety, may be responsible for internally hot fragments.

Competitive C-H and O-D bond fission channels in the UV photodissociation of the deuterated hydroxymethyl radical CH2OD.

Photodissociation studies of the CH2OD radical in the region 28,000-41,000 cm(-1) (357-244 nm), which includes excitation to the 3s, 3p(x), and 3p(z) states, are reported. H and D photofragments are monitored by using resonance-enhanced multiphoton ionization (REMPI) from the onset of H formation at approximately 30,500 cm(-1) to the origin band region of the 3pz(2A")<--1 2A" transition at 41,050 cm(-1). Kinetic energy distributions P(ET) and recoil anisotropy parameters as a function of kinetic energy, beta(eff)(ET), are determined by the core sampling technique for the channels producing H and D fragments. Two dissociation channels are identified: (I) D+CH2O and (II) H+CHOD. The contribution of channel II increases monotonically as the excitation energy is increased. Based on the calculations of Hoffmann and Yarkony [J. Chem. Phys. 116, 8300 (2002)], it is concluded that conical intersections between 3s and the ground state determine the final branching ratio even when initial excitation accesses the 3px) and 3pz states. The different beta(eff) values obtained for channels I and II (-0.7 and approximately 0.0, respectively) are attributed to the different extents of out-of-plane nuclear motions in the specific couplings between 3s and the ground state (of A' and A'' symmetry, respectively) that lead to each channel. The upper limit to the dissociation energy of the C-H bond, determined from P(ET), is D0(C-H)=3.4+/-0.1 eV (79+/-2 kcal/mol). Combining this value with the known heats of formation of H and CH2OD, the heat of formation of CHOD is estimated at DeltaHf(0)(CHOD)=24+/-2 kcal/mol.

Exit channel dynamics in the ultraviolet photodissociation of the NO dimer: (NO)2→NO(A 2Σ+)+NO(X 2Π)

The correlated angular and product rotational state distributions obtained in the 221.67 nm photodissociation of (NO)2 yielding NO(A 2Σ+)+NO(X 2Π) have been examined in the molecular beam using the velocity map ion imaging technique. The translational energy and angular distributions of selected rotational states of NO(A 2Σ+) products in N=0, 5, 6 for which the maximum energies available to the NO(X 2Π) products are 202.5, 142.5, and 118.5 cm−1, respectively, have been measured. The recoil anisotropy parameter of the photofragments, βeff, is 1.2±0.1, less than that previously measured at 213 nm (1.36±0.05). The correlated product state distributions near dissociation threshold agree with the predictions of phase space theory. These experimental results, as well as those obtained previously at 213 nm, are compared to statistical calculations, including v⋅J correlations. Application of the β-ET correlation model to the 213 nm results indicates that [NO(A,N),NO(X,J)] pairs with high NO(X,J) rotational levels are produced preferentially via planar dissociation, in contrast to the statistical expectation of the v⋅J correlation, which reveals no preference for planar dissociation. A mechanism involving vibrational predissociation with restricted intramolecular vibrational energy redistribution can explain both the observed scalar and vector properties. Specifically, the low frequency torsional (out-of-plane) mode does not couple efficiently to the other modes, especially at higher excess energies when the dissociation is rapid. On the other hand, the long-range attraction between NO(A) and NO(X), which is revealed both in the photodissociation dynamics of the dimer and in the quenching of NO(A) by NO(X), encourages long-range mode couplings and can explain the largely statistical rotational state distributions observed near threshold. From images obtained near threshold, the bond energy of the NO dimer in the ground state is determined to be 710±10 cm−1, in good agreement with previous results.

O–D bond dissociation from the 3s state of deuterated hydroxymethyl radical (CH2OD)

The photodissociation of the deuterated hydroxymethyl radical CH2OD is investigated on the lowest excited state, the 3s Rydberg state, in the wavelength region 365–318 nm where the D atom is the only significant product. The translational energy distribution and kinetic energy-dependent anisotropy parameter of the D channel are determined by the core-sampling time-of-flight technique at 352.5 nm. The negative recoil anisotropy parameter βeff=−0.7±0.1 is consistent with the perpendicular nature of the transition from the ground state. The fraction of the available energy partitioned into the translational degree of freedom is 0.69. Ground state products CH2O (1 1A1)+D constitute the main photodissociation channel, and no significant H product is detected at these wavelengths. Comparison with the conical intersection calculations of Hoffman and Yarkony suggests that O–D bond rupture involves crossing from the 3s potential energy surface to a repulsive region of the ground state surface at a large O–D bond distance. Isomerization of CH2OD to CH2DO is not competitive with the fast O–D dissociation.

State-to-state photodissociation of OCS (ν2=0,1|JlM). I. The angular recoil distribution of CO (X 1Σ+;v=0|J)

State-to-state photodissociation experiments of OCS at 230 nm are reported using hexapole state selection of the parent molecule and velocity map ion imaging of the angular recoil of the CO photofragment. The role of the initial rovibrational state (ν2=0,1|JlM) of OCS on the angular recoil distribution is investigated. The CO (X 1Σ+;v=0|J) rotational distribution as well as the angular recoil anisotropy parameter β of the CO photofragment are reported for dissociation of single rovibrational (ν2=0,1|JlM) quantum states of OCS. A strong dependence of the anisotropy parameter β on the initial bending state, ν2=0 or 1, of OCS is observed. The effects of the initial bending state of OCS are rationalized in terms of the strong angular dependence of the transition dipole moment function of OCS for the 1 1Σ−(1 1A″) and 1 1Δ(2 1A′) excited state surfaces involved in the dissociation at 230 nm. The state-to-state imaging experiment provides a revised and improved determination of the binding energy of OCS (ν1,ν2,ν3=0,0,0|J=0)→CO (X 1Σ+;v=0|J=0)+S (1D2), D0=(4.284±0.009) eV.

NO angular distributions in the photodissociation of (NO)2 at 213 nm: Deviations from axial recoil

Angular distributions of selected rotational states of NO(A 2Σ+,ν=0) products obtained in the 213 nm photodissociation of (NO)2 have been determined in a molecular beam by using the photofragment ion imaging technique. Specifically, images of NO(A,ν=0) products in N=0, 11, and 19 have been recorded, for which the maximum energies available to the NO(X 2Π) products are 2038, 1774, and 1278 cm−1, respectively. The recoil anisotropy parameter of the photofragments, βeff, decreases significantly at low center-of-mass translational energies from its maximum value of 1.36±0.05, and depends strongly on the rotational angular momentum of the photoproducts. This behavior is described well by a classical model that takes into account the transverse recoil component mandated by angular momentum conservation. For each of the observed NO(A) N states, highly rotating NO(X) levels are produced via planar dissociation, and the angular momenta are established at an interfragment separation of about 2.6 Å. For most of the center-of-mass translational energy range, both corotating and counterrotating fragments are produced, but at the lowest energies, only the latter are allowed. The correlated rotational energy distributions exhibit deviations from the behavior predicted by phase space theory, suggesting that exit-channel dynamics beyond the transition state influences the product state distributions. In this study, a new method for image reconstruction is employed, which gives accurate angular distributions throughout the image plane.

Ion imaging studies of Cl(2P3/2) fragments arising in the visible photolysis of BrCl: Measurement of orientation, alignment, and alignment-free anisotropy parameters

The photodissociation dynamics of jet-cooled BrCl molecules have been investigated at four different wavelengths in the range 425–485 nm by high-resolution velocity map ion imaging. Four images of the Cl(2P3/2) atomic fragments are recorded at each photolysis wavelength with the probe laser polarization, respectively, linearly aligned and vertical (i.e., perpendicular to the detection axis), right circularly polarized, horizontally linearly polarized (i.e., parallel to the detection axis) and left circularly polarized on successive laser shots, thereby ensuring automatic mutual self-normalization. Appropriate linear combinations of these images allow quantification of the angular momentum alignment of the Cl(2P3/2o) fragments [i.e., the correlation between their recoil velocity (v) and their electronic angular momentum (J)] in terms of the alignment anisotropy parameters s2, α2, η2, and γ2, and determination of the “alignment-free” recoil anisotropy parameter, β0, as a function of parent excitation wavelength. Both incoherent and coherent contributions to the alignment are identified, with both simultaneous parallel and perpendicular excitations to the B 3Π(0+) and C 1Π(1) states and excitations to the Ω=±1 components of the C state contributing to the latter. The deduced values of the alignment-free β parameters indicate (wavelength dependent) contributions from both parallel and perpendicular parent absorptions in this wavelength range. Such a conclusion accords with approximate deconvolutions of the parent absorption spectrum that are currently available, and with determinations of the orientation parameter γ1′ obtained by fitting the difference image obtained when using left and right circularly polarized radiation to probe the ground state Cl atoms arising in the 480.63 nm photodissociation of BrCl when the photolysis laser radiation is polarized linearly at 45° to the detection axis.

Quantum theory of vector correlations in vibrationally mediated photodissociation

The quantum treatment of the photodissociation of aligned, vibrationally excited molecules prepared by polarized laser excitation is presented. A formal expression for the angle-dependent multipole moments of a photofragment is derived. As a specific example, the projection of the laboratory angular distribution (zero-order moment) along a probe direction is considered, and Doppler-shift-dependent fluxes for various arrangements of the polarization directions of the vibrational excitation and photolysis lasers are computed. The profiles depend upon the photodissociation dynamics only through the conventional recoil anisotropy parameter β, as in one-photon dissociation. More generally, the dynamical information obtainable from measurement of vector correlations in VMP is the same as that which can be extracted in a one-photon dissociation experiment. Nevertheless, measurement of vector correlations for vibrationally mediated photodissociation can provide new information on the dissociation dynamics since different regions of the excited potential energy surface(s) are accessed from those accessed in one-photon dissociation.

Nanosecond and femtosecond probing of the dynamics of the UV-photodissociation of perfluoroethyliodide C2F5I

The ns photodissociation of perfluoroethyliodide C2F5I at 266 nm has been studied by using the resonant two-photon ionization (R2PI) technique. Recoil anisotropy parameters as well as average translational energy of the I atoms in the fine structure states P1/22 and P3/22 have been determined. The main contribution (99%) to the absorption at 266 nm was found to be caused by a parallel transition to the Q03 state which gives mainly excited-state atoms I(2P1/2). The ground-state atoms I(2P3/2) were found to appear mainly (88%) from the primarily excited Q03 state via curve-crossing Q03–Q11 and to a lesser extent (12%) from direct absorption by a perpendicular transition to the Q11 and Q13 states. The fs pump–dump technique in combination with ns R2PI probing of the fragments I(2P1/2) and I(2P3/2) and time-of-flight mass spectrometry have been applied to probe the early stage dynamics of the C2F5I molecule on the excited state Q03 potential energy surface (PES). The evolution time of the excited molecule to the point where the energy gap between the excited state Q03 and the ground-state potential energy surfaces drops to a value of about 12 440 cm−1 was found to be 52±13 fs. This time corresponds to about 0.8 Å extension of the C–I bond distance. The molecular dynamics simulation with DFT calculated ground-state PES and Q03 PES with the shape calculated for methyl iodide found in the literature gives reasonable agreement with the experimental result for the evolution time.

Predissociation of the Hydroxymethyl Radical in the 3pz Rydberg State: Formaldehyde + Hydrogen Atom Channel

The photodissociation of the hydroxymethyl radical to hydrogen atom and formaldehyde was investigated following excitation to the origin band of the Rydberg 2A"(3pz) state of jet cooled CH2OD and CD2OH. D and H products were detected by 2-color laser ionization. The peaks in the photofragment yield spectrum of the D product from CH2OD correspond to peaks in the absorption spectrum, and this establishes hydroxymethyl as the source of the observed D atoms. D atoms appear as major products from CH2OD, but the H photofragment yield is not discernible above the background. On the other hand, in the dissociation of CD2OH, only H atoms are produced. It is concluded that isomerization to the methoxy radical is not important. Time-of-flight distributions of the D atom from CH2OD were obtained under core sampling conditions. The product translational energy distribution derived from the data is broad, indicating that the formaldehyde cofragment is produced in the ground electronic state but with an internal energy distribution that extends to the thermochemical limit. The recoil anisotropy parameter was estimated by comparing the intensity of the D signals obtained with parallel and perpendicular polarization of the photolysis laser. The observed anisotropic angular distribution suggests that the dissociation is fast, as also indicated by the line width. The results can be rationalized by a mechanism that involves nonadiabatic transitions from the initially excited Rydberg state to the ground state, with the final crossing occurring in a region of the potential energy surface that leads to direct O−D(H) fission without isomerization. Exit channel dynamics is probably responsible for the high vibrational excitation of the formaldehyde fragment.

Product quantum-state-dependent anisotropies in photoinitiated unimolecular decomposition

Angular distributions of state-selected NO and O products in the photoinitiated unimolecular decomposition of jet-cooled NO2 have been measured by using both the photofragment ion imaging technique with velocity map imaging and ion time-of-flight translational spectroscopy. The recoil anisotropy parameter of the photofragments, β, depends strongly on the rotational angular momentum of the photoproduct. O(3Pj=2,0) angular distributions are recorded at photolysis wavelengths 371.7, 354.7, and 338.9 nm. At these wavelengths, respectively, vibrational levels v=0, v=0,1 and v=0–2 of NO are generated. In addition, β values for NO(v=2) in specific high rotational levels are determined at ∼338 nm. The experimental observations are rationalized with a classical model that takes into account the transverse recoil component mandated by angular momentum conservation. The model is general and applicable in cases where fragment angular momentum is large, i.e., a classical treatment is justified. It is applied here both to the experimental NO2 results, and results of quantum calculations of the vibrational predissociation of the Ne–ICl van der Waals complex. It is concluded that deviations from the limiting β values should be prominent in fast, barrierless unimolecular decomposition, and in certain dissociation processes where a large fraction of the available energy is deposited in rotational excitation of the diatom. The application of the model to NO2 dissociation suggests that the nuclear dynamics leading to dissociation involves a decrease in bending angle at short internuclear separations followed by a stretching motion. This interpretation is in accord with recent theoretical calculations.

Fragment recoil anisotropies in the photoinitiated decomposition of HNCO

The photofragment ion imaging technique is used to determine product recoil anisotropy parameters, β, and correlated state distributions in the S1(1A″)←S0(1A′) photoinitiated decomposition of HNCO into three competing channels: (1) N3H+CO, (2) H+NCO, and (3) N1H+CO [where NH3 and NH1 denote NH(X 3Σ−) and NH(a 1Δ), respectively]. In particular, the region in the vicinity of the N1H+CO threshold is investigated. The measured recoil anisotropies fall into two distinct groups corresponding to time scales of &amp;lt;1 ps (β&amp;lt;−0.6), and &amp;gt;5–10 ps (β≅0.0). With 230.1 nm photolysis, CO(J=0–14) originating in channel (3) is produced with β=−0.8±0.05 via direct dissociation on S1 above a barrier of 470±60 cm−1. CO at low J-states appears with most of the available energy in the translational degree of freedom and is correlated with NH1 in its lowest rotational states. A small contribution to channel (3) from S0 dissociation (observed mainly for J=14,15) gives rise to an isotropic recoil distribution, and a hotter correlated NH1 rotational distribution. At the same wavelength, CO correlated with NH3 is identified by its high translational energy and exhibits an isotropic angular distribution. We propose that the pathway leading to its formation is S1→S0→T1. H-atom signals from channel (2) have isotropic angular distributions at photolysis wavelengths 243−215 nm; this places a lower limit of 8140 cm−1 on the barrier to direct dissociation on S1 to channel (2). The &amp;gt;5 ps time scale for the appearance of channel (2) implies dissociation on S0 following internal conversion. The mechanism described here for the one-photon decomposition of HNCO in the wavelength region 260-230 nm is in accord with other available experimental and theoretical findings.

Resonance enhanced multiphoton ionization time-of-flight study of CF2I2 photodissociation

The resonance enhanced multiphoton ionization time-of-flight (REMPI-TOF) technique was applied to study the complex dissociation of CF2I2 in a supersonic expansion. Using nanosecond excitation at 248, 266, and 304 nm and probing the I3/2 and/or I1/2 photoproducts, we determined the photofragment speed distribution, the recoil anisotropy parameter β and the branching ratio for each of the decay channels. At 248 nm the dissociation proceeds according to a concerted three-body decay, CF2I2→hvCF2+I1/2+I3/2 (90%) or CF2I2→hvCF2+I3/2+I3/2 (10%) consistent with our previous results obtained from photofragment translational spectroscopy. At 304 nm three competing decay channels were found to be operative. In decreasing order of branching, these are: two-body decay to CF2I +I1/2, sequential three-body decay CF2I2→ lim hvCF2I + I3/2→CF2 + I3/2+I3/2, and a concerted three-body decay yielding CF2 +I3/2+I3/2. The positive, almost maximum β values observed for all the primary steps in the 248, 266 and 304 nm photodissociation imply that the excited states which subsequently lead to dissociation are exclusively of B1 symmetry and that the primary bond-breaking processes occur on a subpicosecond time scale.

Doppler Spectroscopy of OH Fragments from the Photodissociation of HOCl at 266 and 355 nm

Photofragment Doppler spectroscopy has been used to characterize the electronically excited states of HOCl in the ultraviolet region. In the photolysis of HOCl, the Doppler profile of the nascent OH fragments shows a marked difference in the recoil anisotropy parameters: β = 1.31 ± 0.12 at 266 nm and −0.70 ± 0.12 at 355 nm. Photoabsorption at 266 nm is characterized by the transition moment predominantly parallel to the O−Cl axis, which is consistent with assignment of the strong band at 250 nm to the 21A‘−11A‘ transition. On the other hand, the transition moment at 355 nm is perpendicular to the O−Cl bond. The measurement of the rotational alignment also shows the transition moment to be perpendicular to the molecular plane at 355 nm, indicating the existence of the 11A‘‘ ← 11A‘ transition above 300 nm.

Photodissociation Dynamics of C2H2, C2D2, and C2HD at 121.6 nm

The product translational energy distribution P(Et) and the recoil anisotropy parameter β(Et) for acetylene (and isotopic variants) photodissociation at 121.6 nm have been obtained by detecting the H(D) atom fragment using the Doppler-selected time-of-flight (TOF) technique. A strong propensity toward the formation of the C2H(Ã 2Π) fragment was revealed. The resolution of this technique is sufficiently high to resolve the vibrational structures of a small polyatomic molecule. The vibrational spectroscopic constants of the C−H and C−C stretching modes for the C2H(Ã 2Π) state were obtained for the first time, and the mode-specific vibrational state distributions were deduced. It is conjectured that two distinct dissociation pathways, arising from the Rydberg−valence surface intersections, are involved in the title processes. The branching of these two pathways is governed by the initial absorption step.

Photodissociation of NO2 at 355 and 351 nm Investigated by Photofragment Translational Spectroscopy

The photodissociation of jet-cooled NO:! at 355 and 351 nm has been investigated by polarized high-resolution photofragment translational spectroscopy. The translational energy distributions P(&) of the nascent photofragment pairs NO + 0 were derived from the measured time-of-flight (TOF) distributions. By comparison of P(ET) with the available energy, the population of vibrationally excited NO was determined to be P(v=O) = 62 f 3% and P(v=l) = 38 F 3% at 355 nm and P(v=O) = 57 f 3% and P(v=l) = 43 F 3% at 351 nm. These findings are consistent with the trend predicted by statistical models of the dissociation process. Nonstatistical decay dynamics are, however, indicated for the rotational degrees of freedom as manifested by bimodal (or multimodal) rotational distributions of NO(v=O, 1). The recoil anisotropy parameter p as a function of fragment translational energy was obtained from the polarized TOF spectra and was found to depend on the photolysis wavelength and on the vibrational state of the NO product: p(v=O) = 1.42 and p(v=l) = 1.25 at 355 nm, whereas p(v=O) = 1.77 and p(v=l) = 1.48 at 351 nm. The NO rotational alignment Af) measured by the laser-induced fluorescence method is at high J values close to the theoretical (perpendicular-type) limit of -0.4 at both photolysis wavelengths. This result, in conjunction with the values of p, implies that the photon absorption occurs via the :!B:! - electronic transition and that the photodissociation takes place essentially without anisotropy loss and hence on a subpicosecond time scale. The observed dependence of p on the product vibrational state suggests two different decay pathways with quantum mechanical effects playing an important role.

Photodissociation of ClNO in the A band

The photodissociation of ClNO at 248 nm has been studied by photofragment translational spectroscopy. The nascent Cl and NO fragments exhibit a recoil anisotropy parameter β=1.2 and their kinetic energy distribution has a distinctly bimodal shape. The main component (80%) peaks at a total translational energy of 50 kcal/mol whereas the weaker one is centered at 35 kcal/mol. The present results, in conjunction with those of previous investigations at 248, 236 and 193 nm, have allowed us to draw a consistent picture of the photodissociation of ClNO, according to which the absorption band “A” consists of three overlapping electronic transitions of symmetry type A′←A′. Each of these transitions produces photofragments with a characteristic angular and energy distribution.

Photodissociation of nitric acid in a cold molecular beam at 193 nm

The photodissociation of HONO 2 at 193 nm has been investigated by photofragment translational spectroscopy. Two competing primary dissociation processes were found: (1) HONO 2→OH + NO 2 (60%) and (2) HONO 2→HONO + O ( 1D) (40%). The translational energy distribution of reaction (1) shows two components which correspond to formation of stable and unstable (hot) NO 2 photofragments with a ration of 1:2. The decay of hot NO 2 produces the secondary fragments NO + O( 3P). The recoil anisotropy parameters associated with the respective dissociation processes β(1) =−0.6±0.1 and β(2) =−0.6±0.2 are consistent with an electronic transition dipole moment μ of the excitation which is oriented parallel to the line connecting the two terminal O atoms in the parent molecule.