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We present theoretical elastic and electronic excitation cross sections and experimental electronic excitation cross sections for electron collisions with pyrimidine. We use the R-matrix method to determine elastic integral and differential cross sections and integral inelastic cross sections for energies up to 15 eV. The experimental inelastic cross sections have been determined in the 15-50 eV impact energy range. Typically, there is quite reasonable agreement between the theoretical and experimental integral inelastic cross sections. Calculated elastic cross sections agree very well with prior results. [ABSTRACT FROM AUTHOR]

The binding energies of the p-, m-, and o-difluorobenzene-H2O complexes have been measured by velocity map ion imaging to be 922±10, 945±10, and 891±4 cm-1, respectively. The lack of variation provides circumstantial evidence for water binding to the three isomers via the same interaction, viz. an in-plane O–H...F hydrogen bond to one of the fluorine atoms on the ring, with a second, weaker interaction of the water O atom with an ortho hydrogen, as determined previously for the p-difluorobenzene-H2O complex [Kang et al., J. Phys. Chem. A 109, 767 (2005)]. The ground state binding energies for the difluorobenzene-H2O complexes are ∼5%–11% larger than that for benzene-H2O, where binding occurs to the π electrons out-of-plane. However, in the S1 state the binding energies of the o- and p-difluorobenzene-H2O complexes are smaller than the benzene-H2O value, raising an interesting question about whether the geometry at the global energy minimum remains in-plane in the excited electronic states of these two complexes. Recoil energy distributions for dissociation of p-difluorobenzene-H2O have been measured from the 31, 52, and 3151 levels of the excited electronic state. These levels are 490, 880, and 1304 cm-1, respectively, above the dissociation threshold. Within the experimental uncertainty, the recoil energy distributions are the same for dissociation from these three states, with average recoil energies of ∼100 cm-1. These recoil energies are 60% larger than was observed for the dissociation of p-difluorobenzene-Ar, which is a substantially smaller increase than the 400% seen in a comparable study of dissociation within the triplet state for pyrazine-Ar, -H2O complexes. The majority of the available energy is partitioned into vibration and rotation of the fragments. [ABSTRACT FROM AUTHOR]

Previously reported dramatic changes in photoelectron angular distributions (PADs) as a function of photoelectron kinetic energy following the ionization of S1 p-difluorobenzene are shown to be explained by a shape resonance in the b2g symmetry continuum. The characteristics of this resonance are clearly demonstrated by a theoretical multiple-scattering treatment of the photoionization dynamics. New experimental data are presented which demonstrate an apparent insensitivity of the PADs to both vibrational motion and prepared molecular alignment, however, the calculations suggest that strong alignment effects may nevertheless be recognized in the detail of the comparison with experimental data. The apparent, but unexpected, indifference to vibrational excitation is rationalized by considering the nature of the resonance. The correlation of this shape resonance in the continuum with a virtual π* antibonding orbital is considered. Because this orbital is characteristic of the benzene ring, the existence of similar resonances in related substituted benzenes is discussed. [ABSTRACT FROM AUTHOR]

Velocity map imaging has been used to measure the distributions of translational energy released in the dissociation of p-difluorobenzene--Ar van der Waals complexes from the &5¹sline;, &3¹sline;, &5²sline;, &3¹5¹sline;, &5³sline;, &3²sline;, and &3²5¹sline; states. These states span 818-3317 cm-1 of vibrational energy and correspond to a range of energies above dissociation of 451-2950 cm-1. The translational energy release (recoil energy) distributions are remarkably similar, peaking at very low energy (10-20 cm-1) and decaying in an exponential fashion to approach zero near 300 cm-1. The average translational energy released is small, shows no dependence on the initial vibrational energy, and spans the range 58-72 cm-1 for the vibrational levels probed. The average value for the seven levels studied is 63 cm-1. The low fraction of transfer to translation is qualitatively in accord with Ewing's momentum gap model [G. E. Ewing, Faraday Discuss. 73, 325 (1982)]. No evidence is found in the distributions for a high energy tail, although it is likely that the experiment is not sufficiently sensitive to detect a low fraction of transfer at high translational energies. The average translational energy released is lower than has been seen in comparable systems dissociating from triplet and cation states. [ABSTRACT FROM AUTHOR]

The translational energy release distribution for dissociation of benzene--Ar has been measured and, in combination with the 601 rotational contour of the benzene product observed in emission, used to determine the rotational J,K distribution of 00 benzene products formed during dissociation from &6¹sline;. Significant angular momentum is transferred to benzene on dissociation. The 00 rotational distribution peaks at J=31 and is skewed to low K:Javerage=27, |K|average=10.3. The average angle between the total angular momentum vector and the unique rotational axis is determined to be 68°. This indicates that benzene is formed tumbling about in-plane axes rather than in a frisbeelike motion, consistent with Ar "pushing off" benzene from an off-center position above or below the plane. The J distribution is very well reproduced by angular momentum model calculations based on an equivalent rotor approach [A. J. McCaffery, M. A. Osborne, R. J. Marsh, W. D. Lawrance, and E. R. Waclawik, J. Chem. Phys. 121, 1694 (2004)], indicating that angular momentum constraints control the partitioning of energy between translation and rotation. Calculations for p-difluorobenzene--Ar suggest that the equivalent rotor model can provide a reasonable prediction of both J and K distributions in prolate (or near prolate) tops when dissociation leads to excitation about the unique, in-plane axis. Calculations for s-tetrazine--Ar require a small maximum impact parameter to reproduce the comparatively low J values seen for the s-tetrazine product. The three sets of calculations show that the maximum impact parameter is not necessarily equal to the bond length of the equivalent rotor and must be treated as a variable parameter. The success of the equivalent rotor calculations argues that angular momentum constraints control the partitioning between rotation and translation of the products. [ABSTRACT FROM AUTHOR]

The dissociation dynamics of p-difluorobenzene-Ar and p-difluorobenzene-Ar[sup +] have been investigated from the &5¹macr; level in S[sub 1] and the 29[sub 2] level in D[sub 0], respectively. The technique of velocity map imaging has been used to determine the translational energy release distributions. In the case of &5¹macr; p-difluorobenzene-Ar, dispersed fluorescence spectra provide the distribution of vibrational energy in the p-difluorobenzene fragment. A significant fraction of the p-difluorobenzene products are formed in the 0[sup 0] level. From the translational energy release data the rotational energy distribution within 0[sup 0] can be inferred. The results show that the average rotational energy is 380 cm[sup -1], >5 times the average translational energy of 70 cm[sup -1]. This rotational excitation infers that dissociation occurs with the Ar atom significantly displaced from its equilibrium position above the center of the aromatic ring. From the average rotational energy it is determined that the Ar atom is, on average, displaced by 1.8-3.7 Å from the center of the aromatic ring at dissociation, i.e., the Ar atom is beyond the carbon atoms. In the case of dissociation from the 29[sub 2] level of p-difluorobenzene-Ar[sup +], the vibrational distribution within the p-difluorobenzene[sup +] product is not known, however it can be inferred from previous studies of dissociation within S[sub 1]. As for the &5¹macr; p-difluorobenzene-Ar case, the evidence suggests that dissociation leads to significant rotational excitation of p-difluorobenzene[sup +]. There are a limited number of destination vibrations within the p-difluorobenzene and p-difluorobenzene[sup +] fragments for dissociation from &5¹macr; (S[sub 1]) and 29[sub 2] (D[sub 0]), respectively. Hence there are only a few, widely separated, values for the combined translational and rotational energy available. Despite this, the translational energy release distributions... [ABSTRACT FROM AUTHOR]

The technique of velocity map imaging has been used to determine the dissociation energies of the van der Waals complexes p-difluorobenzene–Ar and p-difluorobenzene–Kr. The values determined for the S[sub 0], S[sub 1], and D[sub 0] states, respectively, are 337±4, 367±4, and 572±6 cm[sup -1] for p-difluorobenzene–Ar and 398±7, 445±7, and 720±6 cm[sup -1] for p-difluorobenzene–Kr. An ionization potential of 73 549±4 cm[sup -1] for p-difluorobenzene–Kr has been determined by velocity map imaging of photoelectrons. The dissociation energies determined here are inconsistent with dispersed fluorescence spectra of the complexes when these are assigned in the usual way. The issue is that spectra for levels below dissociation show bands where free p-difluorobenzene emits, suggesting that dissociation is occurring from these levels. For the dispersed fluorescence and velocity map imaging results to be consistent, these fluorescence bands must arise from transitions of the van der Waals complexes shifted such that they appear at the free p-difluorobenzene wavelengths. It is proposed that these bands are due to emission from highly excited van der Waals modes populated by intramolecular vibrational redistribution from the initially excited level. From calculations performed for the related benzene–Ar system [B. Fernandez, H. Koch, and J. Makarewicz, J. Chem. Phys. 111, 5922 (1999)], the emitting levels are most likely above the barrier separating different p-difluorobenzene–partner configurations. The fluorescence observations are consistent with those of other techniques if the p-difluorobenzene–partner interaction is the same in the ground and excited electronic states for such highly excited levels. Emission then occurs at the p-difluorobenzene monomer position since the energy shift is the same for the initial and final states. Deducing van der Waals binding energies from t... [ABSTRACT FROM AUTHOR]

Laser photoelectron spectra is obtained following the preparation of 10 vibrational states in S1 p-fluorotoluene. In all cases the vibrational peaks in the photoelectron spectra are assigned, and the spectra are used to deduce the frequencies of 12 vibrational modes in the ion.

The technique of velocity map imaging has been used to determine the dissociation energy for the process p-difluorobenzene–Ar2→p-difluorobenzene–Ar+Ar. The aim of the experiment was to obtain a value for comparison with the dissociation energy for the process p-difluorobenzene–Ar→p-difluorobenzene+Ar in order to ascertain the extent of three-body effects. We find that the dissociation energy for the loss of an Ar atom from p-difluorobenzene–Ar2 is 339±3 cm−1, which is the same within experimental uncertainty as that for the loss of Ar from p-difluorobenzene–Ar (337±4 cm−1). We conclude that the presence of the first Ar affects the binding energy of the second by ∼2% or less. [Copyright &y& Elsevier]

Previously reported dramatic changes in photoelectron angular distributions (PADs) as a function of photoelectron kinetic energy following the ionization of S1 p-difluorobenzene are shown to be explained by a shape resonance in the b(2g) symmetry continuum. The characteristics of this resonance are clearly demonstrated by a theoretical multiple-scattering treatment of the photoionization dynamics. New experimental data are presented which demonstrate an apparent insensitivity of the PADs to both vibrational motion and prepared molecular alignment, however, the calculations suggest that strong alignment effects may nevertheless be recognized in the detail of the comparison with experimental data. The apparent, but unexpected, indifference to vibrational excitation is rationalized by considering the nature of the resonance. The correlation of this shape resonance in the continuum with a virtual pi* antibonding orbital is considered. Because this orbital is characteristic of the benzene ring, the existence of similar resonances in related substituted benzenes is discussed.

Velocity map imaging has been used to measure the distributions of translational energy released in the dissociation of p-difluorobenzene-Ar van der Waals complexes from the 5(1), 3(1), 5(2), 3(1)5(1), 5(3), 3(2), and 3(2)5(1) states. These states span 818-3317 cm(-1) of vibrational energy and correspond to a range of energies above dissociation of 451-2950 cm(-1). The translational energy release (recoil energy) distributions are remarkably similar, peaking at very low energy (10-20 cm(-1)) and decaying in an exponential fashion to approach zero near 300 cm(-1). The average translational energy released is small, shows no dependence on the initial vibrational energy, and spans the range 58-72 cm(-1) for the vibrational levels probed. The average value for the seven levels studied is 63 cm(-1). The low fraction of transfer to translation is qualitatively in accord with Ewing's momentum gap model [G. E. Ewing, Faraday Discuss. 73, 325 (1982)]. No evidence is found in the distributions for a high energy tail, although it is likely that the experiment is not sufficiently sensitive to detect a low fraction of transfer at high translational energies. The average translational energy released is lower than has been seen in comparable systems dissociating from triplet and cation states.

Photoelectron angular distributions (PADs) have been measured following the excitation of the S1 origin band in paradifluorobenzene using a range of ionizing wavelengths and for resolved ion vibrational states. The PADs show a dramatic sensitivity to the photoelectron kinetic energy over an energy range of at least 1 eV from threshold, and almost no sensitivity to any prepared intermediate state alignment. This has important consequences for those studies of intramolecular dynamics that use PADs. We suggest that the observed behavior is caused by a shape resonance in the continuum.