Probing the O2 (a 1Delta g) photofragment following ozone dissociation within the long wavelength tail of the Hartley band

The technique of resonance enhanced multiphoton ionization (REMPI) has been used in conjunction with time-of-flight mass spectrometry (TOFMS), to investigate the dynamics of ozone photolysis in the long wavelength region of the Hartley band (301-311 nm). Specifically, both the translational anisotropy and the rotational angular momentum orientation of the O(2) (a (1)Delta(g); nu=0, J=16-20) fragments have been measured as a function of photolysis wavelength. Within this region, the thermodynamic thresholds for the formation of these products in combination with O ((1)D(2)) are approached and passed, and consequently these studies have allowed an investigation into the effects on the dynamics of slowing fragment recoil velocities and the increasing importance of vibrationally mediated photolysis. The determined beta parameters for all the J states probed follow a similar trend, decreasing from a value typical for the initial (1)B(2)<--(1)A(1) excitation responsible for the Hartley band [for example, beta=1.40+/-0.12 for the O(2) (a (1)Delta(g); J=18) fragment], to a much lower value beyond the thermodynamic threshold for the fragment's production (for example, beta=0.63+/-0.19 for the J=18 fragment following photolysis at 311 nm). This trend, similar to that observed when probing the atomic fragment in a previous set of experiments, [Horrocks et al., J. Chem. Phys. 125, 133313 (2006); Denzer et al., Phys. Chem. Chem. Phys. 16, 1954 (2006)] is consistent with the photodissociation of vibrationally excited ozone molecules beyond the threshold wavelengths and we estimate approximately 1/3 of this to be from excitation in the nu(3) asymmetric stretching mode. These observations are substantiated by the values of the beta(0) (2)(2,1) orientation moment measured, which for photolysis at 301 nm are negative, indicating that a bond opening mechanism provides the key torque for the departing O(2) fragment. The orientation moment becomes positive again for photolysis beyond threshold, however, as the increasing impulsive dissociation again begins to dominate the nature of the rotation of the departing molecular fragment. In addition, a (2+2) REMPI scheme has been utilized to probe the O(2) (a (1)Delta(g)) "low" J fragments, where the majority of the population resides following photolysis within this region. The REMPI-TOFMS technique has been used to confirm the rotational character of a spectral feature through examination of the signal line shapes obtained using different experimental geometries. The dynamical information subsequently obtained, probing the "low" J O(2) (a (1)Delta(g)) fragments on these rotational transitions, has unified previous translational anisotropy results obtained by detecting the O ((1)D(2)) atomic fragment with data for the O(2) (a (1)Delta(g); J=16-20) fragments.     

Photolysis wavelength dependence of the translational anisotropy and the angular momentum polarization of O2(a 1Deltag) formed from the UV photodissociation of O3

The translational anisotropy and angular momentum polarization of the O(2)(a (1)Delta(g),v = 0;J = 15-27) molecular photofragment produced from the UV photodissociation of O(3) in the range from 270 to 300 nm have been determined using resonance-enhanced multiphoton ionization in conjunction with time-of-flight mass spectrometry. At the shortest photolysis wavelengths used, the fragments exhibit the anisotropic vector correlations expected from a prompt dissociation via the (1)B(2) <--(1)A(1) transition. Deviations from this behavior are observed at longer photolysis wavelengths with, in particular, the angular momentum orientation showing a significant reduction in magnitude. This indicates that the dissociation can no longer be described by a purely impulsive model and a change in geometry of the dissociating molecule is implied. This observation is substantiated by the variation of the translational anisotropy with photolysis wavelength. We also observe that the bipolar moments describing the angular momentum polarization of the odd J states probed are consistently lower in magnitude than those of the even J states and that this variation is observed for all photolysis wavelengths.     

Angular distributions and angular momentum alignment of O((3)P(J)) atoms formed in the photolysis of O(2)via the Herzberg continuum

Sliced velocity-map imaging has been used to measure photofragment scattering distributions for the O((3)P(2)) and O((3)P(1)) products of O(2) photolysis following laser excitation into the Herzberg continuum between 205 and 241 nm. The images were analysed to extract the photofragment spatial anisotropy parameter, β, together with the alignment parameters a(∥), a(⊥), a(⊥), and Re[a(∥, ⊥)]. Our alignment measurements bridge the gap between the recent 193 nm measurement of Brouard et al., Phys. Chem. Chem. Phys., 2006, 8, 5549 and those of Alexander et al., J. Chem. Phys., 2003, 118, 10566 at 222 and 237 nm, and extend out to the threshold at 241 nm. Our measured parameters show no strong dependence on photolysis wavelength. Near the threshold we were able to separate the contributions from the O((3)P(2)) + O((3)P(2)) and O((3)P(2)) + O((3)P(1)) channels, and found significantly different photofragment alignments for the two cases.     

Further investigation of the photodissociation dynamics of dichlorocarbene near 248 nm

A further investigation of the 248 nm photodissociation of CCl(2), which expands upon our original study of this process [S. K. Shin and P. J. Dagdigian, Phys. Chem. Chem. Phys. 8, 3446 (2006)], is presented. The CCl(2) parent molecule and the CCl photofragment were detected by laser fluorescence excitation in a molecular beam experiment. From the dependence of the CCl(2) signals on the photolysis laser fluence, attenuation cross sections of the 0(0), 1(1), and 2(1) vibrational levels were determined; the cross sections for the excited vibrational levels were found to be significantly smaller than those for the ground vibrational level. The previously observed fragment CCl bimodal rotational state distribution was found to arise from the photolysis of more than one parent molecule. At low CHCl(3) mole fractions in the gas supplied to the pyrolysis beam source, it was concluded that CCl(2) is the photolysis precursor for both low-J and high-J CCl fragments. On the basis of the dependence of the CCl signals on the photolysis laser fluence, ground and vibrationally excited CCl(2), respectively, were assigned as the precursors to these two classes of fragments. The photofragment excitation spectra for low-J and high-J CCl fragments from the photolysis of CCl(2) were recorded in the wavelength range around 248 nm; both were found to be structureless. The 248 nm photodissociation dynamics of CCl(2) is discussed in light of our experimental observations and quantum chemical calculations of the CCl(2) excited electronic states.     

Cl2O photochemistry: ultraviolet/vis absorption spectrum temperature dependence and O(3P) quantum yield at 193 and 248 nm

The photochemistry of Cl(2)O (dichlorine monoxide) was studied using measurements of its UV/vis absorption spectrum temperature dependence and the O((3)P) atom quantum yield, Φ(Cl(2)O)(O)(λ), in its photolysis at 193 and 248 nm. The Cl(2)O UV/vis absorption spectrum was measured over the temperature range 201-296 K between 200 and 500 nm using diode array spectroscopy. Cl(2)O absorption cross sections, σ(Cl(2)O)(λ,T), at temperatures <296 K were determined relative to its well established room temperature values. A wavelength and temperature dependent parameterization of the Cl(2)O spectrum using the sum of six Gaussian functions, which empirically represent transitions from the ground (1)A(1) electronic state to excited states, is presented. The Gaussian functions are found to correlate well with published theoretically calculated vertical excitation energies. O((3)P) quantum yields in the photolysis of Cl(2)O at 193 and 248 nm were measured using pulsed laser photolysis combined with atomic resonance fluorescence detection of O((3)P) atoms. O((3)P) quantum yields were measured to be 0.85 ± 0.15 for 193 nm photolysis at 296 K and 0.20 ± 0.03 at 248 nm, which was also found to be independent of temperature (220-352 K) and pressure (17 and 28 Torr, N(2)). The quoted uncertainties are at the 2σ (95% confidence) level and include estimated systematic errors. ClO radical temporal profiles obtained following the photolysis of Cl(2)O at 248 nm, as reported previously in Feierabend et al. [J. Phys. Chem. A 114, 12052, (2010)], were interpreted to establish a <5% upper-limit for the O + Cl(2) photodissociation channel, which indicates that O((3)P) is primarily formed in the three-body, O + 2Cl, photodissociation channel at 248 nm. The analysis also indirectly provided a Cl atom quantum yield of 1.2 ± 0.1 at 248 nm. The results from this work are compared with previous studies where possible.     

One-dimensional quantum description of the bending vibrations of HCN/CNH for high values of the total angular momentum

For high values of the quantum number of the total angular momentum J (up to J = 20), quantum mechanical eigenstates (eigenvalues and eigenfunctions) are calculated by the method of Gatti et al. (J. Mol. Spectrosc. 181 (1997) 403) for the bending deformations of HCN and CNH. In particular, we have examined the l-type resonances in highly excited rovibrational states within the framework of a one-dimensional model, i.e. along the reaction pathway for the isomerization reaction HCN/CNH. The potential energy surface used is that of Bowman et al. (J. Chem. Phys. 99 (1993) 308).     

Vector properties of the O(1D2) fragment produced from the photolysis of ozone in the wavelength range of 298 to 320 nm

The speed averaged translational anisotropy and electronic angular momentum polarization of the O(1D2) atomic fragment formed from the photodissociation of ozone in the atmospherically important long wavelength region of the Hartley band (298 to 320 nm) have been measured using resonance enhanced multiphoton ionization time of flight mass spectrometry. The translational anisotropy parameter, beta, is found to decline from 1.1 for photolysis at 300 nm to a minimum value of 0 at 310 nm which is the threshold for production of O(1D2) in conjunction with the O2(a 1Deltag v = 0) molecular cofragment. For photolysis wavelengths greater than 310 nm, O(1D2) is formed from the dissociation of internally excited ozone molecules. The corresponding beta parameters are markedly lower than for atomic fragments produced with the same speed from the photolysis of ground state ozone molecules. This result is consistent with two different pathways contributing to the photolysis of internally excited ozone at the longest wavelengths studied corresponding to initial internal excitation either in the symmetric or asymmetric stretching vibration. In addition, the polarization of the atomic angular momentum has been determined with the incoherent polarization parameters a0(2)(||) and a0(2)(_|) increasing from values of -0.53 and -0.62 at 300 nm to -0.37 and -0.19 at 317 nm, consistent with the increasing contribution from the photolysis of internally excited ozone as the dissociation wavelength lengthens. Evaluation of these alignment parameters allows the populations of the magnetic substrates, mj, to be determined. For example, for a photolysis wavelength of 303 nm the populations of mj = 0, +/- 1, +/- 2 are in the ratio of 0.36: 0.56: 0.08 and this ratio is essentially independent of the photolysis wavelength. The coherent contribution to the atomic polarization is quantified by the Re{a1(2)(||, _|)} and Im{a1(1)(||, _|)} parameters and these are found to vary from -0.21 and 0.21 at 300 nm to -0.04 and 0.24 at 313 nm, respectively.     

Influence of collision energy and reagent rotation on the cross sections and product polarizations of the reaction F + HCl

Quasiclassical trajectory calculations have been carried out for the F+HCl reaction in three dimensions on a recent DHSN PES of the ground 1(2)A' electronic state [M. P. Deskevich, M. Y. Hayes, K. Takahashi, R. T. Skodje, and D. J. Nesbitt, J. Chem. Phys. 124, 224303 (2006)]. The effects of the collision energy and the reagent initial rotational excitation on the cross sections and product polarization are studied for the v = 0 and j ≤ 10 states of HCl over a wide collision energy range. It has been found that either the collision energy or the HCl rotational excitation increase remarkably reaction cross sections. The QCT-calculated integral cross sections are in good agreement with previous QM results. A detailed study on product polarization for the title reaction is also performed. The calculated results show that the product rotational angular momentum j' is not only aligned, but also oriented along the direction perpendicular to the scattering plane. The orientation of the HF product rotational angular momentum vector j' depends very sensitively on the collision energy and also affected by the reagent rotation. The theoretical findings and especially the roles of the collision energy and initial rotational momentum on the product polarization are discussed and reasonably explained by the HLH mass combination, the property of the PES, as well as the reactive mechanism.     

The photodissociation dynamics of ozone at 193 nm: an O(1D2) angular momentum polarization study

Polarized laser photolysis, coupled with resonantly enhanced multiphoton ionization detection of O(1D2) and velocity-map ion imaging, has been used to investigate the photodissociation dynamics of ozone at 193 nm. The use of multiple pump and probe laser polarization geometries and probe transitions has enabled a comprehensive characterization of the angular momentum polarization of the O(1D2) photofragments, in addition to providing high-resolution information about their speed and angular distributions. Images obtained at the probe laser wavelength of around 205 nm indicate dissociation primarily via the Hartley band, involving absorption to, and diabatic dissociation on, the B 1B2(3 1A1) potential energy surface. Rather different O(1D2) speed and electronic angular momentum spatial distributions are observed at 193 nm, suggesting that the dominant excitation at these photon energies is to a state of different symmetry from that giving rise to the Hartley band and also indicating the participation of at least one other state in the dissociation process. Evidence for a contribution from absorption into the tail of the Hartley band at 193 nm is also presented. A particularly surprising result is the observation of nonzero, albeit small values for all three rank K = 1 orientation moments of the angular momentum distribution. The polarization results obtained at 193 and 205 nm, together with those observed previously at longer wavelengths, are interpreted using an analysis of the long range quadrupole-quadrupole interaction between the O(1D2) and O2(1Deltag) species.     

Reactivity enhancement of ultracold O(3P)+H2 collisions by van der Waals interactions

The role of van der Waals forces in O((3)P)+H(2)(upsilon=1,j=0) collisions is investigated theoretically at low and ultralow temperatures. Quantum scattering calculations have been performed for zero total angular momentum using the lowest London-Eyring-Polanyi-Sato double-polynomial (3)A(") potential-energy surface reported by [Rogers et al., J. Phys. Chem. A 104, 2308 (2000)] and its recent BMS1 and BMS2 extensions developed by [Brandao et al., J. Chem. Phys. 121, 8861 (2004)] which provide a more accurate treatment of the van der Waals interaction. Our calculations show that van der Waals forces strongly influence chemical reactivity at ultracold translational energies. The presence of a zero-energy resonance for the BMS1 surface is found to enhance reactivity in the ultracold regime and shift the Wigner threshold to lower temperatures.     

Photofragment angular momentum distribution beyond the axial recoil approximation: predissociation

We present the quantum mechanical expressions for the angular momentum distribution of the photofragments produced in slow predissociation. The paper is based on our recent theoretical treatment [J. Chem. Phys. 123, 034307 (2005)] of the recoil angle dependence of the photofragment multipole moments which explicitly treat the role of molecular axis rotation on the electronic angular momentum polarization of the fragments. The electronic wave function of the molecule was used in the adiabatic body frame representation. The rigorous expressions for the fragment state multipoles which have been explicitly derived from the scattering wave function formalism have been used for the case of slow predissociation where a molecule lives in the excited quasibound state much longer than a rotation period. Possible radial nonadiabatic interactions were taken into consideration. The optical excitation of a single rotational branch and the broadband incoherent excitation of all possible rotational branches have been analyzed in detail. The angular momentum polarization of the photofragments has been treated in the high-J limit. The polarization of the photofragment angular momenta predicted by the theory depends on photodissociation mechanism and can in many cases be significant.     

Accurate quantum wave packet calculations for the F + HCl → Cl + HF reaction on the ground 1(2)A' potential energy surface

We present converged exact quantum wave packet calculations of reaction probabilities, integral cross sections, and thermal rate coefficients for the title reaction. Calculations have been carried out on the ground 1(2)A' global adiabatic potential energy surface of Deskevich et al. [J. Chem. Phys. 124, 224303 (2006)]. Converged wave packet reaction probabilities at selected values of the total angular momentum up to a partial wave of J = 140 with the HCl reagent initially selected in the v = 0, j = 0-16 rovibrational states have been obtained for the collision energy range from threshold up to 0.8 eV. The present calculations confirm an important enhancement of reactivity with rotational excitation of the HCl molecule. First, accurate integral cross sections and rate constants have been calculated and compared with the available experimental data.     

Photofragment angular momentum distributions in the molecular frame. III. Coherent effects in the photodissociation of polyatomic molecules with circularly polarized light

We extend the a(q) (k)(s) polarization parameter model [T. P. Rakitzis and A. J. Alexander, J. Chem. Phys. 132, 224310 (2010)] to describe the components of the product angular momentum polarization that arise from the one-photon photodissociation of asymmetric top molecules with circularly polarized photolysis light, and provide a general equation for fitting experimental signals. We show that the only polarization parameters that depend on the helicity of the circularly polarized photolysis light are the A(0) (k) and Re[A(1) (k)] (with odd k) and the Im[A(1) (k)] (with even k); in addition, for the unique recoil destination (URD) approximation [for which the photofragment recoil v arises from a unique parent molecule geometry], we show that these parameters arise only as a result the interference between at least two dissociative electronic states. Furthermore, we show that in the breakdown of the URD approximation (for which the photofragment recoil v arises from a distribution of parent molecule geometries), these parameters can also arise for dissociation via a single dissociative electronic state. In both cases, the A(0) (k) and Re[A(1) (k)] parameters (with odd k) are proportional to cosΔφ, and the Im[A(1) (k)] parameters (with even k) are proportional to sinΔφ, where Δφ is the phase shift (or average phase shift) between the interfering paths so that Δφ can be determined directly from the A(q) (k), or from ratios of these A(q) (k) parameters. Therefore, the determination of these A(q) (k) parameters with circularly polarized photolysis light allows the unambiguous measurement of coherent effects in polyatomic-molecule photodissociation.     

Rovibrational states of the H2O-H2 complex: an ab initio calculation

All bound rovibrational levels of the H(2)O-H(2) dimer are calculated for total angular momentum J = 0-5 on two recent intermolecular potential surfaces reported by Valiron et al. [J. Chem. Phys. 129, 134306 (2008)] and Hodges et al. [J. Chem. Phys. 120, 710 (2004)] obtained through ab initio calculations. The method used handles correctly the large amplitude internal motions in this complex; it involves a discrete variable representation of the intermolecular distance coordinate R and a basis of coupled free rotor wave functions for the hindered internal rotations and the overall rotation of the dimer. The basis is adapted to the permutation symmetry associated with the para/ortho (p/o) nature of both H(2)O and H(2) as well as to inversion symmetry. Dimers containing oH(2) are more strongly bound than dimers with pH(2), as expected, with dissociation energies D(0) of 33.57, 36.63, 53.60, and 59.04 cm(-1)for pH(2)O-pH(2), oH(2)O-pH(2), pH(2)O-oH(2), and oH(2)O-oH(2), respectively, on the potential of Valiron et al. that corresponds to a binding energy D(e) of 235.14 cm(-1). Rovibrational wave functions are computed as well and the nature of the bound states in the four different dimer species is discussed. Converged rovibrational levels on both potentials agree well with the high-resolution spectrum reported by Weida and Nesbitt [J. Chem. Phys. 110, 156 (1999)]; the hindered internal rotor model that was used to interpret this spectrum is qualitatively correct.     

Rotationally correlated reactivity in the CH (v = 0, J, F(i)) + O2 → OH (A) + CO reaction

The rotational-state-selected CH (v = 0, J, F(i)) beam has been prepared by using an electric hexapole and applied to the crossed beam reaction of CH (v = 0, J, F(i)) + O(2) → OH (A) + CO at different O(2) beam conditions. The rotational state selected reactive cross sections of CH (RSSRCS-CH) turn out to depend remarkably on the rotational state distribution of O(2) molecules at a collision energy of ～?0.19 eV. The reactivity of CH molecules in the N = 1 rotational states (namely ∣J = 1∕2, F(2)> and ∣J = 3∕2, F(1)> states, N designates the angular momentum excluding spin) becomes strongly enhanced upon a lowering of the rotational temperature of the O(2) beam. The RSSRCS-CH in these two rotational states correlate linearly with the population of O(2) molecule in the specific K(O(2)) frame rotation number states: CH(|J = 1/2,F(2)>) with O(2)(|K(O(2)) = 1>);CH(|J = 3/2,F(1)>) with O(2)(|K(O(2)) = 3>). These linear correlations mean that the rotational-state-selected CH molecules are selectively reactive upon the incoming O(2) molecules in a specific rotational state; here, we use the term "rotationally correlated reactivity" to such specific reactivity depending on the combination of the rotational states between two molecular reactants. In addition, the steric asymmetry in the oriented CH (∣J = 1∕2,?F(2),?M = 1∕2>) + O(2) (|K(O(2)) = 1>) reaction turns out to be negligible (< ±1%). This observation supports the reaction mechanism as theoretically predicted by Huang et al. [J. Phys. Chem. A 106, 5490 (2002)] that the first step is an intermediate formation with no energy barrier in which C-atom of CH molecule attacks on one O-atom of O(2) molecule at a sideways configuration.     

Parent-molecule rotational depolarization of photofragment angular momentum distributions: diatomic and polyatomic molecules

We extend the A(q)(k) polarization-parameter model, which describes product angular momentum polarization from one photon photodissociation of polyatomic molecules in the molecular frame [J. Chem. Phys., 2010, 132, 224310], to the case of rotating parent molecules. The depolarization of the A(q)(k) is described by a set of rotational depolarization factors that depend on the angle of rotation of the molecular axis γ. We evaluate these rotational depolarization factors for the case of dissociating diatomic molecules and demonstrate that they are in complete agreement with the results of Kuznetsov and Vasyutinskii [J. Chem. Phys., 2005, 123, 034307] obtained from a fully quantum mechanical approach of the same problem, showing the effective equivalence of the two approaches. We further evaluate the set of rotational depolarization factors for the case of dissociating polyatomic molecules that have three (near) equal moments of inertia, thus extending these calculations to polyatomic systems. This ideal case yields insights for the dissociation of polyatomic molecules of various symmetries when we compare the long lifetime limit with the results obtained for the diatomic case. In particular, in the long lifetime limit the depolarization factors of the A(0)(k) (odd k), Re(A(1)(k)) (even k) and Im(A(1)(k)) (odd k) for diatomic molecules vanish; in contrast, for polyatomic molecules the depolarization factors for the A(0)(k) (odd k) reduce to a value of 1/3, whereas for the Re(A(1)(k)) (even k) and Im(A(1)(k)) (odd k) they reduce to 1/5.     

Molecular photofragment orientation in the photodissociation of H2O2 at 193 nm and 248 nm

Angular momentum orientation has been observed in the OH(X(2)Π, v = 0) fragments generated by circularly polarized photodissociation of H(2)O(2) at 193 nm and 248 nm. The magnitude and sign of the orientation are strongly dependent on the OH(X) photofragment rotational state. In addition to conventional laser induced fluorescence methods, Zeeman quantum beat spectroscopy has also been used as a complementary tool to probe the angular momentum orientation parameters. The measured orientation at 193 nm is attributed solely to photodissociation via the ?(1)A state, even though at this wavelength H(2)O(2) is excited near equally to both the ?(1)A and B(1)B electronic states. This observation is confirmed by measurements of the photofragment orientation at 248 nm, where access to the ?(1)A state dominates. Several possible mechanisms are discussed to explain the observed photofragment orientation, and a simple physical model is developed, which includes the effects of the polarization of the parent molecular rotation upon absorption of circularly polarized light. Good agreement between the experimental and simulation results is obtained, lending support to the validity of the model. It is proposed that photofragment orientation arises mainly from the coupling of the parent rotational angular momentum with that induced during photofragmentation.     

The photodissociation dynamics of O2 at 193 nm: an O3PJ angular momentum polarization study

In the following paper we present translational anisotropy and angular momentum polarization data for O((3)P(1)) and O((3)P(2)) products of the photodissociation of molecular oxygen at 193 nm. The data were obtained using polarized laser photodissociation coupled with resonantly enhanced multiphoton ionization and velocity-map ion imaging. Under the jet-cooled conditions employed, absorption is believed to be dominated by excitation into the Herzberg continuum. The experimental data are compared with previous experiments and theoretical calculations at this and other wavelengths. Semi-classical calculations performed by Groenenboom and van Vroonhoven [J. Chem. Phys, 2002, 116, 1965] are used to estimate the alignment parameters arising from incoherent excitation and dissociation and these are shown to agree qualitatively well with the available experimental data. Following the work of Alexander et al. [J. Chem. Phys, 2003, 118, 10566], orientation and alignment parameters arising from coherent excitation and dissociation are modelled more approximately by estimating phase differences generated subsequent to dissociation via competing adiabatic pathways leading to the same asymptotic products. These calculations lend support to the view that large values of the coherent alignment moments, but small values of the corresponding orientation moments, could arise from coherent excitation of (and subsequent dissociation via) parallel and perpendicular components of the Herzberg I, II and III transitions.     

Complete determination of the photoionization dynamics of a polyatomic molecule. I. Experimental photoelectron angular distributions from A1Au acetylene

Angle-resolved photoelectron spectra from rotationally selected A1Au state acetylene have been recorded using velocity-map imaging. Several Renner-Teller split vibrational bands have been observed and assigned, showing good agreement with previous zero kinetic energy photoelectron (ZEKE) work [S. T. Pratt, P. M. Dehmer, and J. L. Dehmer, J. Chem. Phys. 99, 6233 (1993); S.-J. Tang, Y.-C. Chou, J. J.-M. Lin, and Y.-C. Hsu, ibid. 125, 133201 (2006).] The extracted photoelectron angular distributions (PADs) corresponding to these bands show a strong dependence on the vibronic angular momentum projection quantum number K+. Subbands with odd K+ show PADs with maximum intensity along the polarization vector of the ionizing laser beam, while those with even K+ show PADs with maximum intensity perpendicular to this direction. Velocity-map images recorded at low photoelectron energies approach rotational resolution of the ion, and the evolution of the PADs with increasing rotational level prepared in the A1Au state indicates the potential of a "complete" determination of the photoionization dynamics of the A1Au state. This is further investigated in the following paper.