Zero kinetic energy photoelectron spectroscopy of jet cooled benzo[a]pyrene from resonantly enhanced multiphoton ionization

We report zero kinetic energy (ZEKE) photoelectron spectroscopy of benzo[a]pyrene (BaP) via resonantly enhanced multiphoton ionization (REMPI). Our analysis concentrates on the vibrational modes of the first excited state (S(1)) and those of the ground cationic state (D(0)). Similar to pyrene, another peri-condensed polycyclic aromatic hydrocarbon we have investigated, the first two electronically excited states of BaP exhibit extensive configuration interactions. However, the two electronic states are of the same symmetry, hence vibronic coupling does not introduce any out-of-plane modes in the REMPI spectrum, and Franck-Condon analysis is qualitatively satisfactory. The ZEKE spectra from the in-plane modes observed in the REMPI spectrum demonstrate strong propensity in preserving the vibrational excitation of the intermediate state. Although several additional bands in combination with the vibrational mode of the intermediate state are identifiable, they are much lower in intensity. This observation implies that the molecular structure of BaP has a tremendous capability to accommodate changes in charge density. All observed bands of the cation are IR active, establishing the role of ZEKE spectroscopy in mapping out far infrared bands for astrophysical applications.     

Resonantly enhanced multiphoton ionization and zero kinetic energy photoelectron spectroscopy of chrysene: a comparison with tetracene

We report the electronic and vibrational spectroscopy of chrysene using resonantly enhanced multiphoton ionization (REMPI) and zero kinetic energy (ZEKE) photoelectron spectroscopy. As an isomer of tetracene, chrysene contains a kink in the middle of the four fused hexagonal rings, which complicates not just the symmetry but, more importantly, the molecular orbitals and hence vibronic transitions. Incidentally, the two nearby electronically excited states of chrysene have the same symmetry, and vibronic coupling introduces no out-of-plane vibrational modes. As a result, the REMPI spectrum of chrysene contains essentially only in-plane ring deformation modes, similar to that of tetracene. However, density functional calculations using gaussian even after the inclusion of vibronic coupling can only duplicate the observed REMPI spectrum in a qualitative sense, and the agreement is considerably worse than our recent work on a few pericondensed polycyclic aromatic hydrocarbons and on tetracene. The ZEKE spectrum of chrysene via the origin band of the intermediate electronic state S(1), however, can be qualitatively reproduced by a straightforward Franck-Condon calculation. The ZEKE spectra from vibrationally excited states of the S(1), on the other hand, demonstrate some degree of mode selectivity: the overall intensity of the ZEKE spectrum can vary by an order of magnitude depending on the vibrational mode of the intermediate state. A scaling factor in the theoretical vibrational frequency for the cation is also needed to compare with the experimental result, unlike tetracene and pentacene.     

Resonantly enhanced two photon ionization and zero kinetic energy spectroscopy of jet-cooled 4-aminopyridine

We report studies of supersonically cooled 4-aminopyridine (4-AP) using two-color resonantly enhanced multiphoton ionization (REMPI) and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. With the aid of ab initio and density functional calculations, vibrational modes of the first electronically excited state (S1) of the neutral species and those of the cation have been assigned, and the adiabatic ionization potential has been determined to be 62291+/-6 cm(-1). The REMPI spectrum of the S1 state is dominated by ring deformation modes and the inversion mode of the amino group, while the ZEKE spectra demonstrate a strong propensity of Deltav=0, where v is the vibrational quantum number of the intermediate vibronic state from S1. In addition, the ZEKE spectra obtained via different vibrational levels of the S1 state contain four common features, corresponding to the activation of four different vibrational modes of the cation. These observations are explained in terms of the structural changes from the ground state to S1 and further to the cation. The vibrational mode distributions in both the REMPI and the ZEKE spectra, the excitation energy of the S1 state, and the ionization potential of 4-AP, are remarkably similar to those of aniline, suggesting that the electronic activity is centered on the ring.     

Resonantly enhanced multiphoton ionization and zero kinetic energy photoelectron spectroscopy of 2-indanol conformers

We report studies of a supersonically cooled 2-indanol using two-color resonantly enhanced multiphoton ionization (REMPI) and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. In the REMPI experiment, we have identified three conformers of 2-indanol and assigned the vibrational structures of the first electronically excited state for the two major conformers. Conformer Ia contains an intramolecular hydrogen bond between the -OH group and the phenyl ring, while conformer IIb has the -OH group in the equatorial position. We have further investigated the vibrational spectroscopy of the cation for the two major conformers using the ZEKE spectroscopy. The two conformers display dramatically different vibrational distributions. The ZEKE spectrum of conformer Ia shows an extensive progression in the puckering mode of the five member ring, indicating a significant geometry change upon ionization. The ZEKE spectra of conformer IIb are dominated by single vibronic transitions, and the intensity of the ZEKE signal is much stronger than that of conformer Ia. These results indicate an invariance of the molecular frame during ionization for conformer IIb. We have performed ab initio and density functional theory calculations to obtain potential energy surfaces along the dihedral angle involving the -OH group for all three electronic states. In addition, we have also calculated the vibrational distribution of the ZEKE spectrum for the puckering mode of the five member ring. Not only the vibrational frequencies but also the intensity distributions for both conformers have been reproduced satisfactorily. The adiabatic ionization energies have been determined to be 68 593+/-5 cm(-1) for conformer Ia and 68 981+/-5 cm(-1) for conformer IIb.     

Observation of rotamers of m-aminobenzoic acid: zero kinetic energy photoelectron and hole-burning resonantly enhanced multiphoton ionization spectroscopy

We report studies of supersonically cooled m-aminobenzoic acid using two-color resonantly enhanced multiphoton ionization (REMPI) and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. Two conformers have been identified and characterized using the hole-burning method in the REMPI experiment. With the aid of ab initio and density functional calculations, vibrational modes of the first electronically excited state (S(1)) of the neutral species and those of the ground state cation (D(0)) have been assigned, and the adiabatic ionization potentials have been determined for both conformers. The REMPI spectra are dominated by in-plane motions of the substituents and ring deformation modes. A propensity of Deltav=0, where Deltav is the change in vibrational quantum number from the S(1) to the D(0) state, is observed in the ZEKE spectra. The origin of this behavior is discussed in the context of electron back donation from the two substituents in the excited state and in the cationic state. Comparisons of these results with those of p-aminobenzoic acid will be analyzed.     

Zero kinetic energy photoelectron spectroscopy of p-amino benzoic acid

We report studies of supersonically cooled p-amino benzoic acid using one-color resonantly enhanced multiphoton ionization and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. With the aid of ab initio and density functional calculations, vibrational modes of the first electronically excited state S(1) of the neutral species and those of the cation have been assigned, and the adiabatic ionization potential has been determined to be 64 540+/-5 cm(-1). A common pattern involving the activation of five vibrational modes of the cation is recognizable among all the ZEKE spectra. A propensity of Deltav=0, where v is the vibrational quantum number of the intermediate vibronic state from S(1), is confirmed, and the origin of this behavior is discussed in the context of electron back donation from the two substituents in the excited state and in the cationic state. A puzzling observation is the doublet splitting of 37 cm(-1) in the ZEKE spectrum obtained via the inversion mode of the S(1) state. This splitting cannot be explained from our density functional calculations.     

3s Rydberg and cationic States of pyrazine studied by photoelectron spectroscopy

We have studied 3s(n-1 and pi-1) Rydberg states and D0(n-1) and D1(pi-1) cationic states of pyrazine [1,4-diazabenzene] by picosecond (2 + 1) resonance-enhanced multiphoton ionization (REMPI), (2 + 1) REMPI photoelectron imaging, He(I) ultraviolet photoelectron spectroscopy (UPS), and vacuum ultraviolet pulsed field ionization photoelectron spectroscopy (VUV-PFI-PE). The new He(I) photoelectron spectrum of pyrazine in a supersonic jet revealed a considerably finer vibrational structure than a previous photoelectron spectrum of pyrazine vapor. We performed Franck-Condon analysis on the observed photoelectron and REMPI spectra in combination with ab initio density functional theory and molecular orbital calculations to determine the equilibrium geometries in the D0 and 3s(n-1) states. The equilibrium geometries were found to differ slightly between the D0 and 3s states, indicating the influence of a Rydberg electron on the molecular structure. The locations of the D1-D0 and 3s(pi-1)-3s(n-1) conical intersections were estimated. From the line width in the D1 <-- S0 spectrum, we estimated the lifetime of D1 to be 12 fs for pyrazine and 15 fs for fully deuterated pyrazine. A similar lifetime was estimated for the 3s(pi-1) state of pyrazine by REMPI spectroscopy. The vibrational feature of D1 observed in the VUV-PFI-PE measurement differed dramatically from that in the UPS spectrum, which suggests that the high-n Rydberg (ZEKE) states converging to the D1 vibronic state are short-lived due to electronic autoionization to the D0 continuum.     

The role of symmetry and optical selection rules in revealing the molecular structure of the lowest Rydberg and ionic states of the 1,4-diazabicyclo[2.2.2]octane-Arn (n = 1,2,3) van der Waals complexes

The 1,4-diazabicyclo[2.2.2]octane-Arn (n = 1,2,3) van der Waals complexes (DABCO-Arn) have been investigated using a combination of (1 + 1') resonance enhanced multiphoton ionization (REMPI) and zero electron kinetic energy (ZEKE) spectroscopy. The additivity of the spectral shifts observed in both REMPI and ZEKE spectra, taken together with analysis of vibrational structure, suggest that in both DABCO-Ar and DABCO-Ar2 the argon atoms bind in equivalent equatorial (face) locations between two adjacent (CH2)2 bridges. However, the cumulative evidence from both REMPI and ZEKE spectra, together with ab initio results, suggests that the DABCO-Ar3 complex does not revert to D3h symmetry, but rather adopts a C2v structure in which all three argon atoms bind to one side of the DABCO framework. The exceptionally low wave-number vibrational structure observed in the REMPI spectra suggest that the van der Waals interaction in the excited state is extremely weak. However, ionization necessarily increases the strength of the interaction by virtue of the introduction of charge-induced dipole forces, as revealed by a consistent increase in vibrational wave numbers of the modes observed in the resultant ZEKE spectra.     

Zero kinetic energy spectroscopy of hydroquinone-water (1:1) complex: a probe for conformer assignment

Zero kinetic energy (ZEKE) photoelectron spectroscopy of the hydroquinone-water (HQW) complex was carried out to characterize its S(1)-S(0) resonantly enhanced multiphoton ionization (REMPI) spectrum in terms of the cis and trans conformers. The ZEKE spectra of the hydroquinone isomers show differences in the Franck-Condon (FC) activity of a few ring modes, viz., modes 15, 9b, and 6b, due to the different symmetries of the two isomers. These modes were used as a "diagnostic tool" to carry out the categorical assignment of the REMPI spectrum of the HQW complex. It was found that the FC activity of these diagnostic modes in the cationic ground state (D(0)) of the water complex is similar as that of the monomer. The two lowest energy transitions in the REMPI spectrum of the water complex, 33,175 and 33,209 cm(-1), were reassigned as the band origins of the cis and trans hydroquinone-water complexes, which is opposite of the previous assignment. The intermolecular stretching mode (sigma) of the complex shows a long progression, up to v(')=4, in the cationic ground state and is strongly coupled to other observed ring modes. The Franck-Condon factors for different members in the progression were calculated using the potential energy surfaces computed ab initio. These agree well with the observed intensity patterns in the progression. The ionization potential of the trans and cis complexes was determined to be 60,071+/-4 and 60,024+/-4 cm(-1), respectively.     

The weak hydrogen bond in the fluorobenzene-ammonia van der Waals complex: Insights into the effects of electron withdrawing substituents on pi versus in-plane bonding

The fluorobenzene-ammonia van der Waals complex has been studied using a combination of two-color resonance enhanced multiphoton ionization (REMPI) spectroscopy, counterpoise corrected RICC2 ab initio molecular orbital calculations, and multidimensional Franck-Condon analysis. The experimental REMPI spectrum is characterized by a dominant, blueshifted band origin, and weak activity in intermolecular vibrational modes. RICC2 geometry optimizations and numerical vibrational frequency calculations of the neutral ground and first excited states have been performed on a number of different structural isomers of the complex using basis sets ranging from augmented double-zeta to quadruple-zeta level. Ground state basis set superposition error corrected zero-point binding energies show the in-plane sigma complex, forming a pseudo-six-membered ring connecting the fluorine atom and ortho-hydrogen, to be consistently the most stable of all six conformations considered, at all levels of theory. Comparison of computed zero-point excitation energies for the most stable pi and sigma conformers with fluorobenzene show that the sigma complex is the only conformer predicted to exhibit a spectral blueshift upon electronic excitation. The computed neutral ground and first excited state geometries and frequencies were used to perform multidimensional Franck-Condon simulations of the S(1)-S(0) vibronic spectrum for each of the most stable conformers. These simulations yielded null spectra for transitions involving the most stable of the pi complexes, pi(bridge); a spectrum rich in strong intermolecular vibrational structure for the second of the pi complexes, in complete contrast to the experimental spectrum; and for the sigma complex, a spectrum exhibiting weak intermolecular activity in line with that observed experimentally. This last simulation allowed an almost complete vibrational assignment of the intermolecular structure in the REMPI spectrum. The agreement between computational results and experiment overwhelmingly favors assignment of the spectrum to the in-plane sigma complex.     

Laser-induced fluorescence spectroscopy of NC3S

In a discharged supersonic jet of acetonitrile and carbon disulfide, we have for the first time observed an electronic transition of the NC(3)S radical using laser-induced fluorescence (LIF) spectroscopy. A progression originating from the C-S stretching mode of the upper electronic state appears in the excitation spectrum. Each band of the progression has a polyad structure due to anharmonic resonances with even overtones of bending modes. Rotationally resolved spectra have been observed by high-resolution laser scans, and the electronic transition is assigned to A 2Pii-X 2Pii. For the vibronic origin band, the position and the effective rotational constant of the upper level have been determined to be 21 553.874(1) and 0.046 689(4) cm(-1), respectively. The dispersed fluorescence spectrum from the zero vibrational level of A 2Pi3/2 has also been observed; its vibrational structure is similar to that of the LIF excitation spectrum, showing a prominent C-S stretching progression with polyad structures. The vibrational frequencies of the C-S stretching mode in the ground and excited electronic states are determined to be 550 and 520 cm(-1), respectively. Fluorescence decay profiles have been measured for several vibronic levels of the A state.     

Laser induced fluorescence spectroscopy of a mixed dimer between 2-pyridone and 7-azaindole

We report here the laser induced fluorescence excitation (FE) and dispersed fluorescence (DF) spectra of a 1:1 mixed dimer between 7-azaindole (7AI) and 2-pyridone (2PY) measured in a supersonic free jet expansion of helium. Density functional theoretical calculation at the B3LYP/6-311++G** level has been performed for predictions of the dimer geometry and normal mode vibrational frequencies in the ground electronic state. A planar doubly hydrogen-bonded structure has been predicted to be the most preferred geometry of the dimer. In the FE spectrum, sharp vibronic bands are observed only for excitation of the 2PY moiety. A large number of low-frequency vibronic bands show up in both the FE and DF spectra, and those bands have been assigned to in-plane hydrogen bond vibrations of the dimer. Spectral analyses reveal Duschinsky-type mixing among those modes in the excited state. No distinct vibronic band structure in the FE spectrum was observed corresponding to excitations of the 7AI moiety, and the observation has been explained in terms of nonradiative electronic relaxation routes involving the 2PY moiety.     

Ultraviolet absorption and vibrational spectra of 2-fluoro-5-bromopyridine

The ultraviolet absorption spectrum in the range 340-185 nm in the vapour and solution phase has been measured for 2-fluoro-5-bromopyridine. Three fairly intense band systems identified as the pi* <-- pi transitions II, III and IV have been observed. A detailed vibronic analysis of the vapor and solution spectra is presented. The first system of bands is resolved into about sixty-two distinct vibronic bands in the vapour-phase spectrum. The 0,0 band is located at 35944 cm(-1). Two well-developed progressions, in which the excited state frequencies nu'25 (283 cm(-1)) and nu'19 (550 cm(-1)) are excited by several quanta, have been observed. The corresponding excited state vibrational and anharmonicity constants are found to be omega'i = 292 cm(-1), x'ii = 4.5 cm(-1) (i = 25) and omega'i = 563.8 cm(-1), x'ii = 6.9 cm(-1) (i = 19). The other two band systems show no vibronic structure, the band maxima being located at 48346 and 52701 cm(-1), respectively. The oscillator strength of the band systems in different solutions and the excited state dipole moments associated with the first two transitions have been determined by the solvent-shift method. The infrared spectrum in the region 4000-130 cm(-1) and the laser Raman spectrum of the molecule in the liquid state have been measured and a complete vibrational assignment of the observed frequencies is given. A correlation of the ground and excited state fundamental frequencies observed in the UV absorption spectrum with the Raman or infrared frequencies is presented.     

Laser-induced fluorescence and hole-burning spectra of functional dyes in supersonic jets: spectra of a merocyanine dye

In LIF (laser-induced fluorescence) excitation and hole-burning spectra of a merocyanine dye, EtMD, in a supersonic jet, bands due to one of the four possible isomers were observed. Only lower-frequency vibronic bands were observed in the LIF-excitation spectra, while broad and deep dips were observed in the higher energy region of the hole-burning spectra where no LIF bands were observed. The LIF bands were found to consist of two transition systems, which are presumably due to the molecules in the ground and low-lying excited vibrational states. Possible relaxation processes of EtMD in the excited electronic state were discussed.     

S1(1A1)<--S0(1A1) transition of benzo[g,h,i]perylene in supersonic jets and rare gas matrices

The study of the S1(1A1)<--S0(1A1) transition of benzo[g,h,i]perylene (BghiP, C22H12) in supersonic jets and solid rare gas matrices is reported. In the jet-cooled spectrum, the origin band position is located at 25,027.1+/-0.2 cm-1, the assignment being supported by the analysis of vibrational shifts and rotational band contours. Except for the origin band, which is weak, all bands are attributed to the fundamental excitation of nontotally symmetric b1 vibrational modes of S1. The intensity pattern is interpreted as a consequence of the weak oscillator strength of the electronic transition combined with intensity-borrowing through vibronic interaction between the S1(1A1) and S2(1B1) states. The spectra of the S1(1A1)<--S0(1A1) and S2(1B1)<--S0(1A1) transitions have also been measured for BghiP in solid neon and argon matrices. The comparison of the redshifts determined for either transition reveals that the polarizability of BghiP is larger in its S2 than in its S1 state. Bandwidths of 2.7 cm-1 measured in supersonic jets, which provide conditions relevant for astrophysics, are similar to those of most diffuse interstellar bands. The electronic transitions of BghiP are found to lie outside the ranges covered by present databases. From the comparison between experimental spectra and theoretical computations, it is concluded that the accuracy of empirical and ab initio approaches in predicting electronic energies is still not sufficient to identify astrophysically interesting candidates for spectroscopic laboratory studies.     

What is the best DFT functional for vibronic calculations? A comparison of the calculated vibronic structure of the S1-S0 transition of phenylacetylene with cavity ringdown band intensities

The sensitivity of vibronic calculations to electronic structure methods and basis sets is explored and compared to accurate relative intensities of the vibrational bands of phenylacetylene in the S(1)(A(1)B(2)) ← S(0)(X(1)A(1)) transition. To provide a better measure of vibrational band intensities, the spectrum was recorded by cavity ringdown absorption spectroscopy up to energies of 2000 cm(-1) above the band origin in a slit jet sample. The sample rotational temperature was estimated to be about 30 K, but the vibrational temperature was higher, permitting the assignment of many vibrational hot bands. The vibronic structure of the electronic transition was simulated using a combination of time-dependent density functional theory (TD-DFT) electronic structure codes, Franck-Condon integral calculations, and a second-order vibronic model developed previously [Johnson, P. M.; Xu, H. F.; Sears, T. J. J. Chem. Phys. 2006, 125, 164331]. The density functional theory (DFT) functionals B3LYP, CAM-B3LYP, and LC-BLYP were explored. The long-range-corrected functionals, CAM-B3LYP and LC-BLYP, produced better values for the equilibrium geometry transition moment, but overemphasized the vibronic coupling for some normal modes, while B3LYP provided better-balanced vibronic coupling but a poor equilibrium transition moment. Enlarging the basis set made very little difference. The cavity ringdown measurements show that earlier intensities derived from resonance-enhanced multiphoton ionization (REMPI) spectra have relative intensity errors.     

Laser induced fluorescence and resonant two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone

We carried out laser induced fluorescence and resonance enhanced two-color two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone (1-HAQ). The 0-0 band transition to the lowest electronically excited state was found to be at 461.98 nm (21,646 cm(-1)). A well-resolved vibronic structure was observed up to 1100 cm(-1) above the 0-0 band, followed by a rather broad absorption band in the higher frequency region. Dispersed fluorescence spectra were also obtained. Single vibronic level emissions from the 0-0 band showed Stokes-shifted emission spectra. The peak at 2940 cm(-1) to the red of the origin in the emission spectra was assigned as the OH stretching vibration in the ground state, whose combination bands with the C=O bending and stretching vibrations were also seen in the emission spectra. In contrast to the excitation spectrum, no significant vibronic activity was found for low frequency fundamental vibrations of the ground state in the emission spectrum. The spectral features of the fluorescence excitation and emission spectra indicate that a significant change takes place in the intramolecular hydrogen bonding structure upon transition to the excited state, such as often seen in the excited state proton (or hydrogen) transfer. We suggest that the electronically excited state of interest has a double minimum potential of the 9,10-quinone and the 1,10-quinone forms, the latter of which, the proton-transferred form of 1-HAQ, is lower in energy. On the other hand, ab initio calculations at the B3LYP/6-31G(d,p) level predicted that the electronic ground state has a single minimum potential distorted along the reaction coordinate of tautomerization. The 9,10-quinone form of 1-HAQ is the lowest energy structure in the ground state, with the 1,10-quinone form lying approximately 5000 cm(-1) above it. The intramolecular hydrogen bond of the 9,10-quinone was found to be unusually strong, with an estimated bond energy of approximately 13 kcal/mol (approximately 4500 cm(-1)), probably due to the resonance-assisted nature of the hydrogen bonding involved.     

The vibrational structures of furan, pyrrole, and thiophene cations studied by zero kinetic energy photoelectron spectroscopy

The vibrational structures of the electronic ground states ((approximately)X (2)A(2)) of furan, pyrrole, and thiophene cations have been studied by zero kinetic energy (ZEKE) photoelectron spectroscopic method. In addition to the strong excitations of the symmetric a(1) vibrational modes, other three symmetric vibrational modes (a(2), b(1), and b(2)) have been observed unambiguously. These results which cannot be explained by the Franck-Condon principle illustrate that the vibronic coupling and the Coriolis coupling may play important roles in understanding the vibrational structures of the five-membered heterocycle cations. The vibrationally resolved ZEKE spectra are assigned with the assistance of the density function theory calculations, and the fundamental frequencies for many vibrational modes have been determined for the first time. The first adiabatic ionization energies for furan, pyrrole, and thiophene were determined as 8.8863, 8.2099, and 8.8742 eV, respectively, with uncertainties of 0.0002 eV.     

Structure of gas phase radical cation of 1,3,6,8-tetraazatricyclo[4.4.1.1(3,8)] dodecane determined from zero kinetic energy photoelectron spectroscopy

We report gas-phase vibrational spectroscopy of the ground-state cation of 1,3,6,8-tetraazatricyclo[4.4.1.1(3,8)]dodecane (TTD) using two-color two-photon zero kinetic energy photoelectron spectroscopy. From the distribution of active vibrational modes and comparisons between the experiment and theoretical simulation, we offer proof that the cationic state and the first electronically excited state have the same D(2d) symmetry.     

Observation of a New 2§ Excited Electronic State of SF by Resonance-Enhanced Multiphoton Ionization Spectroscopy

The (2+1) resonance-enhanced multiphoton ionization (REMPI) spectrum of SF has been obtained in the single-photon wavelength region of 307-321 nm. Five vibronic bands were observed and assigned to the two-photon transitions from the ground state to a 2§ Rydberg state. The term value Te, vibrational frequency, and the rotational constant of the 2§ Rydberg state were determined. Another 2P state was observed near 312 nm.