Combined theoretical and experimental study of the valence, Rydberg, and ionic states of chlorobenzene.

New photoelectron (PE) and ultra violet (UV) and vacuum UV (VUV) spectra have been obtained for chlorobenzene by synchrotron study with higher sensitivity and resolution than previous work and are subjected to detailed analysis. In addition, we report on the mass-resolved (2 + 1) resonance enhanced multiphoton ionization (REMPI) spectra of a jet-cooled sample. Both the VUV and REMPI spectra have enabled identification of a considerable number of Rydberg states for the first time. The use of ab initio calculations, which include both multi-reference multi-root doubles and singles configuration interaction (MRD-CI) and time dependent density functional theoretical (TDDFT) methods, has led to major advances in interpretation of the vibrational structure of the ionic and electronically excited states. Franck-Condon (FC) analyses of the PE spectra, including both hot and cold bands, indicate much more complex envelopes than previously thought. The sequence of ionic states can be best interpreted by our multi-configuration self-consistent field computations and also by comparison of the calculated vibrational structure of the B and C ionic states with experiment; these conclusions suggest that the leading sequence is the same as that of iodobenzene and bromobenzene, namely: X(2)B1(3b1 (-1)) < A(2)A2(1a2 (-1)) < B(2)B2(6b2 (-1)) < C(2)B1(2b1 (-1)). The absorption onset near 4.6 eV has been investigated using MRD-CI and TDDFT calculations; the principal component of this band is (1)B2 and an interpretation based on the superposition of FC and Herzberg-Teller contributions has been performed. The other low-lying absorption band near 5.8 eV is dominated by a (1)A1 state, but an underlying weak (1)B1 state (πσ(∗)) is also found. The strongest band in the VUV spectrum near 6.7 eV is poorly resolved and is analyzed in terms of two ππ(∗) states of (1)A1 (higher oscillator strength) and (1)B2 (lower oscillator strength) symmetries, respectively. The calculated vertical excitation energies of these two states are critically dependent upon the presence of Rydberg functions in the basis set, since both manifolds are strongly perturbed by the Rydberg states in this energy range. A number of equilibrium structures of the ionic and singlet excited states show that the molecular structure is less subject to variation than corresponding studies for iodobenzene and bromobenzene.

Combined theoretical and experimental study of the valence, Rydberg and ionic states of fluorobenzene.

New photoelectron spectra (PES) and ultra violet (UV) and vacuum UV (VUV) absorption spectra of fluorobenzene recorded at higher resolution than previously, have been combined with mass-resolved (2 + 1) and (3 + 1) resonance enhanced multiphoton ionization (REMPI) spectra; this has led to the identification of numerous Rydberg states. The PES have been compared with earlier mass-analyzed threshold ionization and photoinduced Rydberg ionization (PIRI) spectra to give an overall picture of the ionic state sequence. The analysis of these spectra using both equations of motion with coupled cluster singles and doubles (EOM-CCSD) configuration interaction and time dependent density functional theory (TDDFT) calculations have been combined with vibrational analysis of both the hot and cold bands of the spectra, in considerable detail. The results extend several earlier studies on the vibronic coupling leading to conical intersections between the X(2)B1 and A(2)A2 states, and a further trio (B, C, and D) of states. The conical intersection of the X and A states has been explicitly identified, and its structure and energetics evaluated. The energy sequence of the last group is only acceptable to the present study if given as B(2)B2

Heavy Rydberg behaviour in high vibrational levels of some ion-pair states of the halogens and inter-halogens.

We report the identification of heavy Rydberg resonances in the ion-pair spectra of I2, Cl2, ICl, and IBr. Extensive vibrational progressions are analysed in terms of the energy dependence of the quantum defect δ(Eb) rather than as Dunham expansions. This is shown to define the heavy Rydberg region, providing a more revealing fit to the data with fewer coefficients and leads just as easily to numbering data sets separated by gaps in the observed vibrational progressions. Interaction of heavy Rydberg states with electronic Rydberg states at avoided crossings on the inner wall of the ion-pair potential is shown to produce distinctive changes in the energy dependence of δ(Eb), with weak and strong interactions readily distinguished. Heavy Rydberg behaviour is found to extend well below near-dissociation states, down to vibrational levels ∼18,000-20,000 cm(-1) below dissociation. The rapid semi-classical calculation of δ(Eb) for heavy Rydberg states is emphasised and shows their absolute magnitude to be essentially the volume of phase space excluded from the vibrational motion by avoiding core-core penetration of the ions.

Interpretation of the photoelectron, ultraviolet, and vacuum ultraviolet photoabsorption spectra of bromobenzene by ab initio configuration interaction and DFT computations.

New photoelectron, ultraviolet (UV), and vacuum UV (VUV) spectra have been obtained for bromobenzene by synchrotron study with higher sensitivity and resolution than previous work. This, together with use of ab initio calculations with both configuration interaction and time dependent density functional theoretical methods, has led to major advances in interpretation. The VUV spectrum has led to identification of a considerable number of Rydberg states for the first time. The Franck-Condon (FC) analyses including both hot and cold bands lead to identification of the vibrational structure of both ionic and electronically excited states including two Rydberg states. The UV onset has been interpreted in some detail, and an interpretation based on the superposition of FC and Herzberg-Teller contributions has been performed. In a similar way, the 6 eV absorption band which is poorly resolved is analysed in terms of the presence of two ππ* states of (1)A1 (higher oscillator strength) and (1)B2 (lower oscillator strength) symmetries, respectively. The detailed analysis of the vibrational structure of the 2(2)B1 ionic state is particularly challenging, and the best interpretation is based on equation-of-motion-coupled cluster with singles and doubles computations. A number of equilibrium structures of the ionic and singlet excited states show that the molecular structure is less subject to variation than corresponding studies for iodobenzene. The equilibrium structures of the 3b13s and 6b23s (valence shell numbering) Rydberg states have been obtained and compared with the corresponding ionic limit structures.

Interpretation of the vacuum ultraviolet photoabsorption spectrum of iodobenzene by ab initio computations.

Identification of many Rydberg states in iodobenzene, especially from the first and fourth ionization energies (IE1 and IE4, X(2)B1 and C(2)B1), has become possible using a new ultraviolet (UV) and vacuum-ultraviolet (VUV) absorption spectrum, in the region 29 000-87 000 cm(-1) (3.60-10.79 eV), measured at room temperature with synchrotron radiation. A few Rydberg states based on IE2 (A(2)A2) were found, but those based on IE3 (B(2)B2) are undetectable. The almost complete absence of observable Rydberg states relating to IE2 and IE3 (A(2)A2 and B(2)B2, respectively) is attributed to them being coupled to the near-continuum, high-energy region of Rydberg series converging on IE1. Theoretical studies of the UV and VUV spectra used both time-dependent density functional (TDDFT) and multi-reference multi-root doubles and singles-configuration interaction methods. The theoretical adiabatic excitation energies, and their corresponding vibrational profiles, gave a satisfactory interpretation of the experimental results. The calculations indicate that the UV onset contains both 1(1)B1 and 1(1)B2 states with very low oscillator strength, while the 2(1)B1 state was found to lie under the lowest ππ(∗) 1(1)A1 state. All three of these (1)B1 and (1)B2 states are excitations into low-lying σ(∗) orbitals. The strongest VUV band near 7 eV contains two very strong ππ(∗) valence states, together with other weak contributors. The lowest Rydberg 4b16s state (3(1)B1) is very evident as a sharp multiplet near 6 eV; its position and vibrational structure are well reproduced by the TDDFT results.

The ionic states of iodobenzene studied by photoionization and ab initio configuration interaction and DFT computations.

New valence electron photoelectron spectra of iodobenzene obtained using synchrotron radiation have been recorded. Ionization energies (IEs) determined using multi-configuration SCF calculation (MCSCF) procedures confirmed the adiabatic IE order as: X(2)B1

A combined resonance enhanced multiphoton ionization and ab initio study of the first absorption band of 1,2,4,5-tetrafluorobenzene, pentafluorobenzene, and hexafluorobenzene.

The resonance enhanced multiphoton ionization (REMPI) spectra of jet-cooled penta- and hexafluorobenzene when excited in the region λ(ex) = 265-253 nm of the first absorption band and observed only in the CF(+) mass channel is dominated by rotational structure in the A←X transition of CF. However, structure in the CF(+) channel for λ(ex) > 265 nm is not a continuation of this CF spectrum and is assigned to vibrational activity in two low-frequency modes of a distorted excited state of the parent molecule. The vibrational structure is assigned to the lowest ππ* state from a comparison with the equivalent spectrum of 1,2,4,5-tetrafluorobenzene. Ab initio calculations at the CIS level of theory of the ππ* state of the penta- and hexafluorobenzene reveal a much more distorted equilibrium geometry (C1 symmetry) than that of 1,2,4,5-tetrafluorobenzene. Long progressions observed in the λ(ex) > 265 nm REMPI spectra of C6HF5 and C6F6 are assigned to two very low frequency (~30 and 80 cm(-1)) modes. The role of the close-lying ππ* and πσ* states in determining the energy redistribution of the initially excited state by internal conversion is discussed. Both the fluorescent yield and the direct production of CF(X) are associated with transfer to the lower-lying πσ* state.

A critical re-assignment of the Rydberg states of iodomethane based on new polarization data.

2- and 3-photon excitation of components of the lower Rydberg states of iodomethane (CH3I) using linearly and circularly polarized light, followed by ionization with one more photon, is used to determine their molecular term symbol, Ω, values as well as quantum defects. These Ω values, together with a detailed theoretical analysis, require a re-assignment of the 7s and 8s states to various components of the 5d and 6d states, but there is evidence of (n+2)s∕nd hybridization in the pairs of Ω = 1 states. Predissociation sets in for all Rydberg states beyond 6d based on the ground ((2)Π(3∕2)) state of the core, but sharp autoionizing resonances based on the (2)Π(1∕2) core state are assigned to the 9s, 7d, and 5f states. The dominant effect of the singlet∕triplet character of the Rydberg states on their accessibility from the ground state, seen in bromomethane and chloromethane, is again apparent and a concordant interpretation of the Rydberg spectra of CH3I can now be presented. Evidence for coupling of some Ω = 1 and Ω = 0(+) Rydberg states with a repulsive valence state and an ion-pair state, respectively, is also put forward.

Characterization of the potential minimum of the F '0(u)(+)(1D2) ion-pair state of Cl2 using (1 + 2') optical-optical double resonance excitation and mass-resolved ion detection.

Vibrational levels of the F(')0(u)(+)((1)D(2)), F0(u)(+)((3)P(0)), and D0(u)(+)((3)P(2)) ion-pair states of (35)Cl(2) and (35)Cl(37)Cl in the range 62,500-67,600 cm(-1) have been observed using (1 + 2(')) optical-optical double resonance excitation with mass-resolved ion detection. The strong F(')0(u)(+)((1)D(2))/F0(u)(+)((3)P(0)) coupling has been modelled by a coupled two-state calculation. An optimized fit of the experimental data used an F(')0(u)(+)((1)D(2)) state potential with a T(e) of 65,177 cm(-1) and an R(e) of ≈2.636 Å with a coupling constant of ≈430 cm(-1). The calculation assigns the first observed members of the F(')0(u)(+)((1)D(2)) state progression of (35)Cl(2) and (35)Cl(37)Cl at 64,998 and 65,094 cm(-1), respectively, as transitions to v = 0.

Efficient long-range collisional energy transfer between the E0(g)(+)(3P2) and D0(u)(+)(3P2) ion-pair states of I2, induced by H2O, observed using high-resolution Fourier transform emission spectroscopy.

Using high-resolution Fourier transform emission techniques, we have resolved rotational structure in the D0(u)(+)((3)P(2)) → X0(g)(+) emission following collisional transfer from the E0(g)(+)((3)P(2)) state in I(2). The P:R branch ratios in the E0(g)(+)((3)P(2)) → D0(u)(+)((3)P(2)) transfer are found to vary enormously with v(E) and v(D). We show that the observed intensities are all consistent with the transfer being dominated by long-range, near-resonant collisions with residual H(2)O. Unequal P:R branch ratios in the E0(g)(+)((3)P(2)) → A1(u) emission have been shown to result from mixing of the E0(g)(+)((3)P(2)) and ß1(g)((3)P(2)) states via Ω-uncoupling.

Amplified spontaneous emission and collisional transfer from the f0(g)+ ((3)P0) ion-pair state of I2.

The work presented here extends previous studies of amplified spontaneous emission (ASE) between ion-pair (charge-transfer) states of I(2) and shows that ASE can occur between states correlating with different states of the cation, namely, f0(g)(+)((3)P(0)) and D0(u)(+)((3)P(2)), despite the smaller transition dipole moment between them. A value of 0.34 e A is obtained for the transition dipole under experimental conditions where the f0(g)(+)((3)P(0))-->D0(u)(+)((3)P(2)) ASE is eliminated. No F0(u)(+)((3)P(0))<--f0(g)(+)((3)P(0)) ASE transfer is observed despite the combination of favorable Franck-Condon factors and transition dipoles. The F0(u)(+)((3)P(0))<--f0(g)(+)((3)P(0)) transfer is shown to be purely collisional and a propensity for transfers involving the smallest energy mismatch is observed.

Long-range collisional energy transfer between charge-transfer (ion-pair) states of I2, induced by H2O and I2(X).

Long-range (resonant) energy transfer, between g/u charge-transfer states of molecular iodine [i.e., f0(g) (+)((3)P(0))-->F0(u)(+)((3)P(0)) and E0(g)(+)((3)P(2))-->D0(u) (+)((3)P(2))], induced by collisions with H(2)O and I(2)(X) via multipole coupling, has been observed. Large rate constants, up to 5 x 10(-9) molecules(-1) cm(3) s(-1), for collisional transfer between a range of vibrational levels of the f0(g)(+)((3)P(0)) and F0(u)(+)((3)P(0)) ion-pair states of I(2), by H(2)O, are reported. Some previously reported studies on E0(g)(+)((3)P(2))-->D0(u)(+)((3)P(2)) and f0(g)(+)((3)P(0))-->F0(u)(+)((3)P(0)) collisional transfer, induced by I(2)(X), have been repeated and revised rate data are presented; the range of initially excited vibrational states studied has also been extended. Much smaller rate constants for quenching by I(2)(X), compared to H(2)O, are found and it is proposed that H(2)O desorbed from the walls of the sample cell could have significantly affected much larger rate data previously reported in the literature. For both collision partners, a model is proposed in which long-range, near-resonant interactions can occur when there is close matching of the change in energy in the ion-pair states with the change in energy that accompanies the rotational transition undergone by the collision partner.

Evidence for Rydberg doorway states in photoion pair formation in bromomethane.

The vacuum ultraviolet laser-excited photoion-pair formation spectrum of CH 3Br has been measured under high resolution in the threshold region. The (2 + 1) and (3 + 1) resonance-enhanced multiphoton ionization spectra in the same energy region are also reported. By comparison of the spectra in this and a more extended region, resonances in the photoion-pair formation spectrum are assigned to p and f Rydberg states. It is concluded that all the structure in the photoion-pair formation spectrum near threshold can be accounted for by members of the Omega = 0 subset of Rydberg states that act as doorway states to the ion channel.

Collisional energy transfer in the intermediate states used for optical-optical double resonance excitation of ion-pair states in I2.

The optical-optical double resonance spectra of I(2) and I(2)-Xe mixtures at room temperature reported in the literature using a fixed-wavelength, broad band pump laser have now been recorded using a tuneable, narrow band source. We show that during the time of the overlapped laser pulses ( approximately 10 ns) and with 10-20 Torr of Xe there is widespread collisional energy transfer in the intermediate state and that this phenomenon offers an alternative explanation for the broad bands in the excitation spectrum, assigned to XeI(2) complexes by the authors of the earlier study (M. E. Akopyan, I. Y. Novikova, S. A. Poretsky and A. M. Pravilov, Chem. Phys., 2005, 310, 287). Dispersed emission bands, previously attributed to direct fluorescence from the ion-pair state(s) of the complexes, are re-assigned to emission from ion-pair states of the parent I(2) that are populated by collisional energy transfer out of the initially excited state.

Observation of three weakly bound valence states of I2.

Structured emission in the gas phase to two weakly bound valence states that correlate with the third dissociation limit, I*(2P1/2)+I*(2P1/2), designated as (bb), from two third tier ion-pair states of I2 correlating with I-(1S0)+I+(1D2), the 1g(1D2), and F'0u+(1D2) states, has been observed for the first time. The 1u(bb) state is shown to be bound by 377+/-2 cm(-1) and molecular constants have been determined. Vibrational structure in the 0g+(bb) state could not be resolved but the spectrum is consistent with the state being bound by 435 cm(-1). The relative integrated intensities of the emissions from both ion-pair states to various valence states have also been measured, and some aspects are rationalized in terms of the electronic configurations of the upper and lower states. Bound levels of a previously uncharacterized 1g(ab) valence state have also been observed in emission from the gamma1u(3P2) ion-pair state. The lower state is shown to be bound by 270+/-2 cm(-1) and molecular constants have been determined.

Characterization of a shallow-bound 0g+ valence state of I2 using emission from the D 0u+(3P2) and F' 0u+(1D2) ion-pair states populated by amplified spontaneous emission.

A range of vibrational levels of the D 0(u)(+)((3)P(2)) and F' 0(u)(+)((1)D(2)) ion-pair states of I(2) is shown to be easily generated by amplified spontaneous emission (ASE) from their more accessible partners, E 0(g)(+)((3)P(2)) and f' 0(g)(+)((1)D(2)), in sufficient concentration for dispersed fluorescence studies of the D 0(u)(+)((3)P(2)) --> 0(g)(+)(bb) and F' 0(u)(+)((1)D(2)) --> 0(g)(+)(bb) transitions to be carried out. T(0) (J = 49) of this shallow-bound 0(g)(+)(bb) valence state is unambiguously determined and an improved R(e) value of 3.952 +/- 0.005 A is obtained from optimizing the fit of the intensities of the vibrational progressions in the 0(g)(+)(bb) state, and T(e) is found to be 27311.3 +/- 2 cm(-1), leading to D(e) = 442.0 +/- 2 cm(-1).

The ns and nd Rydberg states of O2 described by Hund's case (e).

Using Hund's case (e) representation, we have obtained a simulation of the 5s Rydberg states of O(2) for J=2 by fitting the experimental data obtained recently by Sheard et al. [J. Chem. Phys. 118, 8781 (2003)]. Our analysis permits us to include evidence of not only the mixing of Hund's case (a) states by spin-orbit interaction, but also by L and S uncouplings. This mixing is even more important for the nd Rydberg states. For the 3d Rydberg state, J=2, we have been able to suggest for the first time an assignment for both the 3d sigma (1)Pi(g) and the 3d delta (1)Pi(g) states.

Electric-field-induced g/u mixing of the E0g+(3P2) and D0u+(3P2) ion-pair states of jet-cooled I2 observed using optical triple resonance.

Electric-field-induced electronic state g/u mixing of nearly isoenergetic rovibrational levels of the E0g+(3P2) and D0u+(3P2) ion-pair states of I2 has been observed using optical triple resonance combined with resonance ionization. Detectable mixing with applied fields of 1 kV/cm occurs over a range of energy level separations of < or = 0.3 cm(-1).