Resonance-enhanced Two-photon Ionization Spectroscopy Research Articles

Nonplanar structure of C6H5SCF3 facilitates πσ∗-mediated photodissociation reaction on the S1 state

Vibrational structure of trifluoromethylthiobenzene (C6H5SCF3) on the S1 state has been investigated by resonance-enhanced two-photon ionization spectroscopy and nature of predissociation dynamics is inferred from homogeneously broadened spectral features. As C6H5SCF3 adopts a nonplanar structure in both the S0 and S1 states, the effective adiabatic barrier generated by avoided crossing of optically-bright bound S1 (ππ∗) and dark-repulsive S2 (πσ∗) surfaces along the reaction coordinate is significantly lowered, giving the S1 lifetime of ∼300fs. This experiment demonstrates that the molecular structure spanned by the reactive flux near the curve-crossing region dictates reaction rate as well as nonadiabatic transition probability.

Resonance-enhanced two-photon ionization spectroscopy and theoretical calculations of 3,5-difluoroanisole and its Ar-containing complex

The structure and vibrations of 3,5-difluoroanisole (3,5-DFA) in the first electronically excited (S1) state were studied by mass-analyzed resonant two-photon ionization (R2PI) technique as well as the quantum chemical calculations. The ab initio and density functional theory (DFT) calculations reveal that only one structure is stable for each of the S0, S1, and D0 states. In the one color R2PI spectrum, the band origin of the S1←S0 electronic transition (00 band) of 3,5-DFA is found to be 37,595±3cm−1. In the S1 state, most of the bands observed are related to the in-plane ring deformation and out-of-plane bending vibrations. The adiabatic ionization energy (IE) of 3,5-DFA is determined to be 70,096±15cm−1 by the two color R2PI technique, in agreement with the values predicted by the DFT approaches. The dihalogen-substitution effects on the molecular structure, vibrational frequencies, and electronic transition and ionization energies were discussed in detail. The van der Waals complex of 3,5-DFA with argon (3,5-DFA···Ar) was also observed and studied. The 00 band of 3,5-DFA···Ar complex is red-shifted by about 9cm−1 with respect to that of 3,5-DFA. Both the experimental data and the calculated results indicate that the formation of 3,5-DFA···Ar complex gives only a weak influence on the properties of 3,5-DFA moiety.

The REMPI spectra of o-, m- and p-bromofluorobenzene and the photodissociation of p-bromofluorobenzene

The vibrations of o-, m- and p-bromofluorobenzene (BrFPh) in the first electronically excited state are studied by (1+1) resonance-enhanced two-photon ionization spectroscopy. The observed vibrational bands have been assigned on the basis of comparison with the results from theoretical calculations. The band origins of the S1←S0 electronic transitions of o-, m- and p-BrFPh, which were influenced by positive resonance effect and negative inductive effect, are found to be 36986.68cm−1, 36961.48cm−1 and 36223.20cm−1 respectively. Meanwhile, the potential barrier height (the energy of the crossing points between the bound S1 state and a repulsive state relative to the S1 minimum) of p-BrFPh is determined to be lower than 2815cm−1 (0.3490eV) by the REMPI spectra of bromine atom and p-BrFPh molecule.

High-Resolution Mass-Selective UV Spectroscopy of Pseudoephedrine: Evidence for Conformer-Specific Fragmentation

Using resonance-enhanced two-photon ionization spectroscopy with mass resolution of jet-cooled molecules, a low-resolution S(1) ← S(0) vibronic spectrum of pseudoephedrine was recorded at the mass channels of three distinct fragments with m/z = 58, 71, and 85. Two of the fragments, with m/z = 71 and 85, are observed for the first time for this molecule. The vibronic spectra recorded at different mass channels feature different patterns, implying that they originate from different conformers in the cold molecular beam, following conformer-specific fragmentation pathways. Highly resolved spectra of all prominent vibronic features were measured, and from their analysis based on genetic algorithms, the molecular parameters of the conformers giving rise to the respective bands have been determined. Comparing the experimental results with those obtained from high-level ab initio quantum chemistry calculations, the observed prominent vibronic bands have been assigned to originate from four distinct conformers. The conformers are separated into two groups that have different fragmentation pathways determined by the different intramolecular interactions.

Resonance-enhanced multiphoton ionization (REMPI) spectroscopy of the 35Cl and 37Cl isotopomers of p-chloroanisole

The geometric structures and vibrations of p-chloroanisole isotopomers in the first electronically excited state were studied by mass-analyzed resonance-enhanced two-photon ionization spectroscopy and by theoretical calculations. The band origins of the S 1 ← S 0 electronic transitions of 35Cl and 37Cl isotopomers were found to be equivalent at 34 859 ± 3 cm −1. Assignments of the observed vibrational bands of the two isotopomers were made mainly based on the CIS/cc-PVDZ calculations and on conformity with the available data in the literature. Although the general spectral features of these two isotopomers are similar, the frequencies of some vibrational modes are different. This frequency shift partially depends on the degree of involvement of the chlorine atom in the molecular vibrations.

Water binding sites in 2-para- and 2-ortho-fluorophenylethanol: A high-resolution UV experiment and ab initio calculations

The singly hydrated complexes of the flexible prototype molecules 2-para-fluorophenylethanol and 2-ortho-fluorophenylethanol have been investigated by combination of high-resolution resonance-enhanced two-photon ionization spectroscopy in a cold supersonic beam and quantum chemistry ab initio calculations. We have identified the conformational structures of the above complexes, which correspond to water binding to the most stable gauche monomer's conformers in both cases. No structural changes of the host molecules upon the attachment of a single water molecule have been found. For the 2-ortho-fluorophenylethanol-water complex we have observed an additional structure with one of the higher-in-energy gauche conformers of the monomer. This corroborates the assumption that the complexation with water stabilizes the higher-energy conformer of the monomer, precluding it from relaxation to the lowest-energy geometry.

Gas-phase conformations and exciton couplings in 5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene

The conformations and exciton couplings in 5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene (THDC) have been studied using resonance-enhanced two-photon ionization spectroscopy in a supersonic jet expansion. It has been estimated from the spectral analysis that 90% of THDC exists in the twist-boat (TB) conformation; the chair (C) conformer constitutes the remaining 10%. Most of the vibronic activity in the spectrum of THDC is associated with the symmetric flapping of the aromatic rings of the TB conformer. The observed S1/S2 exciton splitting of the TB conformer is 100 cm−1. The S1/S2 transition of the C conformer is found to be forbidden. The exciton splittings of the C and TB conformers were estimated by the spectral analysis of two deuterated isotopomers of THDC. The estimated exciton splittings of the C and TB conformers are 14.7 and 101.9 cm−1, respectively. The supramolecular model of bichromophores with identical chromophores at the CIS/6-31+G(d)//HF/6-31+G(d) level of theory predicted electronic coupling energies that are very close to the experimental exciton coupling energies.

Competition between π and σ hydrogen bonds and conformational probing of 2-orthofluorophenylethanol by low-and high-resolution electronic spectroscopy

The flexible model molecule 2-orthofluorophenylethanol has been investigated by laser-induced fluorescence, and low- and high-resolution resonance-enhanced two-photon ionization spectroscopy in combination with high-level ab initio quantum chemistry calculations. One dominant conformation has been identified in the cold molecular beam corresponding to the most stable theoretically predicted gauche structure stabilized by an intramolecular OH...pi hydrogen bond. A tentative assignment of a higher-lying gauche conformer present in the molecular beam separated by high potential barriers from the most stable one has been made. The missing other higher-energy theoretically predicted conformations most likely relax to the most stable ones during the process of the adiabatic expansion. The good agreement between the experimental and theoretical results demonstrates that even in the case of a substitution with an electronegative atom at the ortho position, bringing about a significant redistribution of the electron density in the benzene ring and providing a convenient binding site for the formation of a competing OH...F sigma hydrogen bond, the nonclassical OH...pi bond remains the preferred binding motif for the most stable conformer.

Resonance-enhanced two-photon ionization mass spectroscopy of ephedrine: Indication for a state-selective fragmentation in a flexible molecule

The vibronic structure of the S 1 ← S 0 spectrum of ephedrine was measured by resonance-enhanced two-photon ionization spectroscopy with mass resolution under cold molecular beam conditions. The spectra recorded at four different mass channels, m/ z = 165 (parent), 58, and the hitherto unknown 71, 85 fragment ions show dissimilar vibronic fine structure and the observed mass pattern strongly depends on the selected intermediate vibrational state. This points to an intermediate state-selected process resulting in a different fragmentation mass pattern. Ab initio calculations at the MP2/6-311++G ∗∗ level demonstrate that the AG (a) conformer is more stable by 238 cm −1 than the next stable GG (a) conformer.

Van der Waals Bonding to a Molecule with π-Electron Conjugation: Styrene-Argon Complex Studied by Mass Selective High Resolution R2PI UV Spectroscopy and Ab-initio Calculations

Combining high-resolution mass-selective resonance-enhanced two-photon ionization spectroscopy analysed by genetic-algorithm-based computer-assisted rotational fit and high-level ab initio calculations we were able to determine the structure and the transition moment orientation of the styrene-Ar complex. The results demonstrate that Ar binds to the benzene ring of styrene. The binding site of Ar is 3.42 Å above the benzene-ring plane and is close to the C 6 symmetry axis of the aromatic ring, but slightly shifted to the vinyl group, which implies that the binding pattern is perturbed by the presence of the bonded to the benzene ring vinyl group. The transition moment ratio of the styrene-Ar complex is considerably different from the one of bare styrene this being a purely mass effect stemming from the reorientation of the principal axes of inertia upon the cluster formation. The red shift of the 00 0 origin band of the styrene-Ar2 complex is almost twice that of the single complex, indicating that the red-shift additivity rule holds in this case. On this basis we conclude that the second Ar atom is bound to the opposite side of the benzene ring.

Specific and nonspecific interactions in a molecule with flexible side chain: 2-phenylethanol and its 1:1 complex with argon studied by high-resolution UV spectroscopy

Using high-resolution resonance-enhanced two-photon ionization spectroscopy in combination with genetic-algorithm-based computer-aided rotational fit analysis and ab initio quantum chemistry calculations we determined the conformational structure and transition moment orientation in 2-phenylethanol and its 1:1 clusters with argon. The results clearly demonstrate that the gauche structure of 2-phenylethanol, which is stabilized by the intramolecular pi-hydrogen bond between the folded side chain and the benzene ring, is the most abundant in the cold molecular beam. In this conformer the transition moment is rotated by 18 degrees from the short axis of the aromatic ring. Two distinct 1:1 complexes of 2-phenylethanol with argon in a cis- and trans-configuration with respect to the side chain have been found. Employing the Kraitchman [Am. J. Phys. 21, 17 (1953)] analysis we have found that the structure of the 2-phenylethanol moiety and the orientation of the transition moment do not change after the complexation with argon within the experimental accuracy. From the measured band intensities we conclude that in addition to the dispersion interaction of the argon atom with the aromatic ring a hydrogen-bond-type interaction with the terminal -OH group of the side chain stabilizes the cis-structure of the 1:1 complex of 2-phenylethanol with argon.

Visible and near-infrared photoabsorption spectrum of Li3O: Resonance enhanced two-photon ionization spectroscopy and ab initio calculations

We report the measurement of the photoabsorption spectrum of Li3O using resonance-enhanced two-photon ionization spectroscopy in the energy range between 0.7 and 2.75 eV. Ab initio geometry optimization calculations at the CCSD(T)/6-311+G(d) level of theory are carried out, resulting in a stable D3h ground state symmetry for Li3O. Vertical excitation energies are computed from the CCSD(T) potential, and the flatness of the potential energy surface is analyzed. A comparison of the recorded absorption spectrum with the theoretical predictions allows an assignment of all the observed bands and excited states in terms of a D3h ground state structure. It is argued that the width of the bands is governed by the flat-bottomed shape of the potential energy surface.

A resonant two-color photoionization threshold determination of the ionization energy of the LiOLi molecule

By measuring the ionization thresholds for several R2PI (resonance-enhanced two-photon ionization spectroscopy) rovibronic transitions from 7Li 16O 7Li[ 1Σ + g (0,0,0) ] to 7Li 16O 7Li[ 1 B 1 ], we have determined the ionization energy of the 7Li 16O 7Li molecule: 50 907±4 cm −1 (6.3117±0.0005 eV). This value is similar to, but much more accurate than, previous experimental and ab initio estimates.

Resonance-enhanced two-photon ionization spectroscopy in plasma chromatography

Resonance-enhanced two-photon ionization spectroscopy in plasma chromatography