Perpendicular Transition Research Articles

Conformation-Specific Spectroscopy of 3-Benzyl-1,5-hexadiyne

3-Benzyl-1,5-hexadiyne (BHD) was studied by a combination of methods, including resonance-enhanced-two-photon ionization, UV-UV hole-burning spectroscopy, resonant ion-dip infrared spectroscopy, and rotational band contour analysis. There are five conformations of BHD observed in the expansion with their 1<-- S0 origins occurring at 37520, 37565, 37599, 37605, and 37631 cm(-1). DFT calculations predict six low energy conformations. Conformational assignments have been made by comparison of the experimental infrared spectra in the alkyl and acetylenic CH stretch region to DFT vibrational frequency and infrared intensity calculations. Rotational band contours provided further confirmation of these assignments. The electronic origin shifts of BHD compare favorably to the electronic origin shifts of 5-phenyl-1-pentyne with the exception of one conformation. This conformation is unique in that it is the only structure with both acetylenic groups in the gauche position over the ring. This gauche-gauche conformation exhibits a perpendicular (b-type) transition and produces extensive vibronic coupling reminiscent of symmetric monosubstituted benzenes.

Spectroscopy and Photophysics of Self-Organized Zinc Porphyrin Nanolayers. 1. Optical Spectroscopy of Excitonic Interactions Involving the Soret Band

The photophysical properties of excited singlet states of zinc tetra-(p-octylphenyl)-porphyrin in 5-25-nm-thick films spin-coated onto quartz slides have been investigated by optical spectroscopy. Analysis of the polarized absorption spectra using a dipole-dipole exciton model with two mutually perpendicular transition dipole moments per molecule shows that the films are built from linear aggregates, i.e., stacks with a slipped-deck-of-cards configuration. The molecular planes of the porphyrins in the stacks are found to be perpendicularly oriented with respect to the substrate plane. Assuming a value of 2-3 for the dielectric constant of the film, from the excitonic shift, an angle of 44 degrees +/- 3 degrees and an interplanar distance of 0.35-0.36 nm between adjacent porphyrins are calculated, close to the ground-state geometry in solution. The ordering in these films was further investigated by the effects of various solvents and temperature annealing. Spin-coating from toluene as a solvent results in highly ordered films, and annealing of these films has little effect on their absorption spectra. However, spin-coating from chloroform or pyridine or exposure of the films to these solvents in their vapor phases changes their ordering presumably due to incorporation of residual solvent molecules. Annealing yields absorption spectra identical to those of films spin-coated from toluene. The absorption spectra are insensitive to atmospheric moisture, in contrast to those of zinc tetraphenylporphyrin films lacking octyl substituents.

A comment on Hönl-London factors

The literature on the Hönl-London factors of diatomic molecules, the factors in intensity formulas dependent on the rotational quantum numbers, is somewhat confused for the case of perpendicular transitions. We derive a formula, Eq. (23), for allowed singlet–singlet transitions between levels of definite parity, and show that it is consistent with standard sum rules. As an illustration, the formula is applied to the calculation of lifetimes in the B 1Π u state of Na 2, showing that the corrected formula gives results in good agreement with experimental measurements.

Ultraviolet photodissociation dynamics of the SH radical

Ultraviolet (UV) photodissociation dynamics of jet-cooled SH radical (in X 2pi(3/2), nu"=0-2) is studied in the photolysis wavelength region of 216-232 nm using high-n Rydberg atom time-of-flight technique. In this wavelength region, anisotropy beta parameter of the H-atom product is approximately -1, and spin-orbit branching fractions of the S(3P(J)) product are close to S(3P2):S(3P1):S(3P0)=0.51:0.36:0.13. The UV photolysis of SH is via a direct dissociation and is initiated on the repulsive 2sigma- potential-energy curve in the Franck-Condon region after the perpendicular transition 2sigma(-)-X 2pi. The S(3P(J)) product fine-structure state distribution approaches that in the sudden limit dissociation on the single repulsive 2sigma- state, but it is also affected by the nonadiabatic couplings among the repulsive 4sigma-, 2sigma-, and 4pi states, which redistribute the photodissociation flux from the initially excited 2sigma- state to the 4sigma- and 4pi states. The bond dissociation energy D0(S-H)=29,245+/-25 cm(-1) is obtained.

Velocity Map Imaging Study of the O2 Ion-Pair Production at 17.499 eV: Simultaneous Parallel and Perpendicular Transitions

The fine structure resolved photofragment O(-)((2)P(j)) image from the O(2) ion-pair production at 17.499 eV has been recorded. The branching ratio for producing the low energy spin-orbit O(-)((2)P(3/2)) component to the high energy spin-orbit O(-)((2)P(1/2)) component is 1:0.78 and the optical transitions for them correspond to perpendicular and parallel transitions, respectively. The anisotropy parameters, 1.64 for channel producing O(-)((2)P(1/2)) and -0.35 for O(-)((2)P(3/2)), suggest that the dissociation proceeds via the states with symmetry (3)Sigma(u)(-) and (3)Pi(u), respectively. Although the main mechanisms for the O(2) ion-pair production are the predissociation via the intermediate Rydberg states, the direct dissociation mechanism for the channel producing O(-)((2)P(1/2)) may also be involved.

Photodissociation of HBr on the surface of Ar n clusters at 193 nm

Photodissociation experiments are carried out for single HBr molecules absorbed on the surface of large Ar n clusters with the average size of 〈 n〉 = 159 at the dissociation wavelength of 193 nm. The kinetic energy of the H atom fragments is measured exhibiting peaks at zero and two energies which mark completely caged and unperturbed fragments going into the two spin orbit channels of Br, respectively. The results are compared with Molecular Dynamics simulations which account for the quantum librational delocalization of the H atom in the HBr molecule and the substitutional surface position of the molecule at 27 K. The latter result is obtained from a trajectory study of the pick-up process under realistic conditions. Both channels are populated by mainly perpendicular transitions in agreement with the results for the bare molecule. The high intensity close to the energy of the excited Br ∗ state mainly originates from slowed down dissociation products from the ground state. Calculations based on these realistic initial state conditions give also good agreement with the experimental results for the dissociation wavelength of 243 nm.

Photodissociation of HI and DI: Polarization of atomic photofragments

The complete angular momentum distributions and vector correlation coefficients (orientation and alignment) of ground state I((2)P(32)) and excited state I((2)P(12)) atoms resulting from the photodissociation of HI have been computed as a function of photolysis energy. The orientation and alignment parameters a(Q) ((K))(p) that describe the coherent and incoherent contributions to the angular momentum distributions from the multiple electronic states accessed by parallel and perpendicular transitions are determined using a time-dependent wave packet treatment of the dissociation dynamics. The dynamics are based on potential energy curves and transition dipole moments that have been reported previously [R. J. LeRoy, G. T. Kraemer, and S. Manzhos, J. Chem. Phys. 117, 9353 (2002)] and used to successfully model the scalar (total cross section and branching fraction) and lowest order vector (anisotropy parameter beta) properties of the photodissociation. Predictions of the a(Q) ((K))(p), parameters for the isotopically substituted species DI are reported and contrasted to the analogous HI results. The resulting polarization for the corresponding H/D partners are also determined and demonstrate that both H and D atoms produced can be highly spin polarized. Comparison of these predictions for HI and DI with experimental measurement will provide the most stringent test of the current model for the electronic structure and the interpretation of the dissociation based on noncoupled excited state dynamics.

A dissociative photoionization study of the O2+ B 2Σg− v+ = 2 state

Photoelectron–photoion coincidence spectra have been recorded using synchrotron radiation to study dissociative photoionization in molecular oxygen. The O2+ B 2Σg− v+ = 2 state has been produced either by direct photoionization or by autoionization from Rydberg states belonging to series converging onto the c 4Σu− limit. Coincidences have been detected between the energy analysed photoelectron and the O+ fragment released in the dissociation of the O2+ B 2Σg− v+ = 2 state. A comparison between the observed and the computer simulated O+ time-of-flight peak profiles has enabled the ratio of the cross sections for the degenerate ionization channels to be determined from threshold 35 eV. The O+ peak shape changes in the vicinity of autoionizing resonances and this variation can be attributed to an enhancement in the photoionization cross section for perpendicular transitions.

Photodissociation of laboratory oriented molecules: Revealing molecular frame properties of nonaxial recoil

We report the photodissociation of laboratory oriented OCS molecules. A molecular beam of OCS molecules is hexapole state-selected and spatially oriented in the electric field of a velocity map imaging lens. The oriented OCS molecules are dissociated at 230 nm with the linear polarization set at 45 degrees to the orientation direction of the OCS molecules. The CO(nu=0,J) photofragments are quantum state-selectively ionized by the same 230 nm pulse and the angular distribution is measured using the velocity map imaging technique. The observed CO(nu=0,J) images are strongly asymmetric and the degree of asymmetry varies with the CO rotational state J. From the observed asymmetry in the laboratory frame we can directly extract the molecular frame angles between the final photofragment recoil velocity and the permanent dipole moment and the transition dipole moment. The data for CO fragments with high rotational excitation reveal that the dissociation dynamics is highly nonaxial, even though conventional wisdom suggests that the nearly limiting beta parameter results from fast axial recoil dynamics. From our data we can extract the relative contribution of parallel and perpendicular transitions at 230 nm excitation.

Photodissociation of HCl and DCl: Polarization of Atomic Photofragments

The complete angular momentum distributions and vector correlation coefficients (orientation and alignment) of ground-state Cl(2P3/2) and excited-state Cl(2P1/2) atoms resulting from the photodissociation of HCl have been computed as a function of photolysis energy. Results for the corresponding H atom partner are also calculated and demonstrate that the H-atom produced is highly spin polarized. The theoretical results are determined using a time-dependent wave packet treatment of the dissociation dynamics based on ab initio potential energy curves, spin−orbit couplings, and dipole moments that have been reported previously [Alexander, M. H.; Pouilly, B.; Duhoo, T. J. Chem. Phys. 1993, 99, 1752]. The theoretical orientation and alignment parameters, (p), that describe the coherent and incoherent contributions to the angular momentum distributions from the multiple dissociative states accessed by parallel and perpendicular transitions, are compared to experimental measurements made at 193 nm and excellent agreement is obtained. Theoretical predictions of the (p) parameters for the isotopically substituted species DCl, for which no experiments have yet been carried out, are reported and contrasted to the analogous HCl results. The results for the H atom spin polarizations are discussed in the context of three static models whose strengths and limitations are highlighted.

Photodissociation of hydrogen iodide on the surface of large argon clusters: the orientation of the librational wave function and the scattering from the cluster cage.

A set of photodissociation experiments and simulations of hydrogen iodide (HI) on Arn clusters, with an average size n = 139, has been carried out for different laser polarizations. The doped clusters are prepared by a pick-up process. The HI molecule is then photodissociated by a UV laser pulse and the outgoing H fragment is ionized by resonance enhanced multiphoton ionization in a (2 + 1) excitation scheme within the same laser pulse at the wavelength of 243 nm. The measured time-of-flight spectra are transformed into hydrogen kinetic energy distributions. They exhibit a strong fraction of caged H atoms at zero-kinetic energy and peaks at the unperturbed cage exit for both spin-orbit channels nearly independent of the polarization. At this dissociation wavelength, the bare HI molecule exhibits a strict state separation, with a parallel transition to the spin-orbit excited state and perpendicular transitions to the ground state. The experimental results have been reproduced using molecular simulation techniques. Classical molecular dynamics was used to estimate the HI dopant distribution after the pick-up procedure. Subsequently, quasi-classical molecular dynamics (Wigner trajectories approach) has been applied for the photodissociation dynamics. The following main results have been obtained: (i) The HI dopant lands on the surface of the argon cluster during the pick-up process, (ii) zero-point energy plays a dominant role for the hydrogen orientation in the ground state of HI-Arn surface clusters, qualitatively changing the result of the photodissociation experiment upon increasing the number of argon atoms, and, finally, (iii) the scattering of hydrogen atoms from the cage which originate from different dissociation states seriously affects the experimentally measured kinetic energy distributions.

Simulating quantum interference in a three-level system with perpendicular transition dipole moments

We consider a three-level V-type atomic system with the ground state coupled by a laser field to only one of the excited states, and with the two excited states coupled together by a dc field. Although the dipole moments of the two dipole-allowed transitions are assumed perpendicular, we demonstrate that this system emulates to a large degree a three-level system with parallel dipole moments---the latter being a system that exhibits quantum interference and displays a number of interesting features. As examples, we show that the system can produce extremely large values for the intensity-intensity correlation function, and that its resonance fluorescence spectrum can display ultranarrow lines. The dressed states for this system are identified, and the spectral features are interpreted in terms of transitions among these dressed states. We also show that this system is capable of exhibiting considerable squeezing.

Dynamics of H-atom loss in adenine

Time- and frequency resolved detection of H-atoms was used to investigate the excited-state photophysics and photochemistry of isolated adenine and 9-methyladenine at two excitation wavelengths, 266 nm and 239.5 nm. By comparison of the two molecules it is shown that dissociation of the N9–H bond is an important deactivation pathway in adenine. The appearance time of the H-atoms was shorter than a few nanoseconds, indicating a rapid non-statistical dissociation. From the Doppler profiles a dissociation energy of 393 ± 20 kJ mol−1 was determined for adenine. Typically 40–50% of the excess energy is released as translation. The Doppler profile of the H-atom photofragments from adenine excited at 239.5 nm shows an anisotropic angular distribution and suggests that dissociation occurs in less than a rotational period. An anisotropy parameter β ≈ −0.9 was derived, indicating an almost pure perpendicular transition. Our data are consistent with a H-atom loss in adenine that can be adequately described by a coupling with πσ* excited states as recently suggested in the literature. The Doppler profiles of 9-methyladenine on the other hand are more isotropic. From the width of the profile it is assumed that not only one of the N–H bonds in the amino group is cleaved, but some H-loss also occurs on the methyl group.

Superradiance in Molecular H Aggregates

Direct evidence of superradiance from an organic semiconductor (quaterthiophene) whose molecules are arranged in a H aggregate fashion, is reported. Time resolved photoluminescence measurements show a linear correlation between the radiative lifetime (tau(rad)) of the purely electronic exciton recombination and the inverse of the number (N(C)) of the coherently emitting dipoles, i.e., tau(rad) proportional, variant 1/N(C). These data support the recently developed theoretical models describing the optical properties of H aggregates of rodlike molecules with a nonvanishing component of the perpendicular molecular transition dipole moment.

Photoionization into the dissociation continuum of H 2+(X 2Σ g+) studied by velocity imaging photoionization coincidence spectroscopy

Photoionization into the dissociation continuum of H 2 +(X 2Σ g +) has been studied using the velocity imaging photoionization coincidence method. Asymmetry parameters for the angular distributions of photoelectrons and fragment H + ions have been measured at photon energies of 21.2, 23.1, 26.9 and 40.8 eV. Molecular frame photoelectron angular distributions (MF-PADs) have also been extracted for the parallel and perpendicular transitions at all the photon energies. All the MF-PADs show p-wave shape, which suggests a dominant p-wave contribution in the photoionization cross-sections at all energies.

Spectroscopic characterization of the first singlet (Ã B11) excited state of Li167O7Li

Using laser induced fluorescence (LIF) and resonance enhanced two-photon ionization (R2PI) spectroscopy, several (ν1′,ν2′,ν3′) vibrational bands of the à 1B1(K′=1)←X̃ 1Σg+(0,0,0) perpendicular transition of the Li167O7Li molecule have been rotationally resolved and analyzed to yield effective A′,B′,C′ values. The estimated geometry of the à 1B1 state does not vary with ν1′ (symmetric stretch mode), but θ′ increases and R′ decreases slightly as ν2′ (bending mode) increases. Extrapolation leads to an estimate for the (0,0,0) state of θ0′=105±5°, R0′=1.86±0.04 Å, and for the potential minimum θe′=102±5°, Re′=1.87±0.04 Å. The strongly bent nature of the à 1B1 state is due to promotion of an O−2 p-electron (b1) from the strongly ionic, linear Li+O−2Li+ ground state to an a1 molecular orbital which has Li/Li bonding character. The à 1B1 state thus has an approximately Li+1/2O−1Li+1/2 charge distribution, so that the ionic bonding is less strong than in the linear ground state, where (from this study and an earlier stimulation-emission pumping study) R0″=1.611±0.003 Å. In fact, the Li–Li distance in the à 1B1 state, ∼3.0 Å, is quite similar to that of the Li2+1 ion, so the bonding may be described as that of Li2+1 bound ionically to the O−1 ion.

Role of cyclic sets of transition dipoles in the pump–probe polarization anisotropy: Application to square symmetric molecules and perpendicular chromophore pairs

Theoretical models considering inhomogeneous energy splitting effects are developed to explain impulsive pump–probe polarization anisotropy experiments on degenerate states of square symmetric molecules and chromophore pairs with perpendicular transition dipoles. When detecting the pump-induced change in probe transmission through the sample, the pump–probe signal arises from lost absorption due to ground state depopulation, stimulated emission from the singly excited states, and absorption from the singly excited states to doubly excited states. Including coherence between the singly excited states, the total impulsive initial anisotropy is 2/5 in all cases. In contrast to the first and second contributions, the time dependent excited state absorption anisotropy depends on a cyclic set of four transition dipoles, not just the energetic splitting. Ignoring molecular rotation, inhomogeneous anisotropy dynamics arise only for coupled chromophore pairs and Jahn–Teller distortions of the degenerate state that produce a cyclic set of transition dipoles different from the set for uncoupled direct product states. In all systems treated, a time dependent pump–probe anisotropy requires coupling between excitations.

Photodissociation of O2 via the Herzberg continuum: Measurements of O-atom alignment and orientation

Irradiation of molecular oxygen O2 in the region of the Herzberg continuum between 218 nm and 239 nm results in the production of open-shell photofragments O(3P)+O(3P). Product O(3Pj; j=0,1,2) atoms were ionized using resonantly enhanced multiphoton ionization (2+1 REMPI) near 225 nm and the ions collected in a velocity-sensitive time-of-flight mass spectrometer. By controlling the polarization of the photolysis and ionization radiation, we have measured alignment and orientation parameters of O-atom electronic angular momentum (j) in the molecule frame. The results show alignment from both parallel and perpendicular transitions that are cylindrically symmetric about the velocity (v) of the recoiling O atom. We also observe electronic alignment that is noncylindrically symmetric about v, resulting from coherence between multiply excited dissociative states. Photodissociation with linearly polarized light is shown to produce O atoms that are oriented in the molecule frame, resulting from interference between parallel and perpendicular dissociative states of O2. Semiclassical calculations that include spin–orbit coupling between six excited states reproduce closely the observed polarization.

Perpendicular dissociation ofD2+in intense Ti:sapphire laser pulses

Detailed results of numerical one-dimensional wave-packet simulations are reported for the dynamics of a ${\mathrm{D}}_{2}^{+}$ molecule subjected to separated and combined 400-nm and 800-nm intense laser pulses with their polarization vectors lying either parallel or perpendicular to the internuclear axis of the molecule. Depending on the laser excitation conditions, the model employed involves from four to a maximum of eight electronic channels, comprising molecular states correlating, in the dissociative limit, with the $n=1$ shell of atomic hydrogen and those correlating with the $n=2$ shell. Preliminary results of calculations including molecular rotations within a two-dimensional model are also reported. Overall, calculated fragment kinetic-energy spectra, the main results of the present work, are in accord with the findings of recent experimental works [Talebpour et al., Phys. Rev. A 62, 042708 (2000); Vijayalakshmi et al., ibid. 053408 (2000)], which reported evidences of perpendicular dissociation induced by intense 400-nm and 800-nm laser pulses. These results are thus supportive of the hypothesis made therein of perpendicular transitions to upper molecular excited states, through a ${\ensuremath{\pi}}_{u}$ doorway. Deviations from the predictions of the Floquet schemes proposed in these works are discussed in terms of the laser-driven rotational dynamics of the molecular ion and in terms of bond-softening versus bond-hardening effects.

Influence of the exchange interaction on Ly-α radiation of multicharged ions

A nonperturbative approach to the description of spectral line shapes of Ly-\ensuremath{\alpha} radiation produced by multicharge ion-pair collisions is proposed. A closed expression that describes the central part of the line as well as the wings with an oscillation structure due to the extremum in the difference between the potential-energy curves has been obtained. It has been found that the exchange interaction leads to the formation of two satellites in the red and blue wings of the Ly-\ensuremath{\alpha} perpendicular transitions.