Heavy Rydberg states: large amplitude vibrations.

Extremely large vibrational amplitude (≈8700 a.u.) heavy Rydberg levels in the HH[combining macron]1Σ+g state, located only 25 cm-1 below the ion-pair dissociation limit, are reported. The calculations are done using a hybrid log derivative/multichannel quantum defect approach that accounts for predissociation and is capable of dealing with any number of long-range closed channels, and of providing positions and widths for the heavy Rydberg resonances. In this case, resonance positions can be reproduced qualitatively using a simple diabatic model (however, the resonance widths cannot). Absolute quantum defects are derived for the vibrational series ranging from ν = 0 to ν = 2010. The influence of the Coulomb potential and continuity of heavy Rydberg behavior throughout the 1Σ+g manifold of states is demonstrated.

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.

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.

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.

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.

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.

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).

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.

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.

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).