Franck-Condon Forbidden Research Articles

High-resolution anion photoelectron spectroscopy of cryogenically cooled 4-atom silicon carbides

High-resolution anion photoelectron spectra of cryogenically cooled , , and obtained using slow photoelectron velocity-map imaging (cryo-SEVI) are presented, providing insight into the geometries, energetics, and vibronic structure of the anionic and the neutral clusters. These spectra yield accurate vibrational frequencies for the neutral clusters. They also yield refined adiabatic detachment energies (ADEs) for the ground states of and of 1.5374(6) eV and 1.9019(4) eV, respectively, while the ADE of a low-lying isomer of is found to be 1.9050(7) eV. The cryo-SEVI spectra show that the ground state of is a distorted trapezoid, and represent the first confirmation of the distorted trapezoid structure of , the only low-lying isomer of this cluster with a permanent dipole moment. Additional transitions are observed from two low-lying anion isomers: a linear structure and a rhombus. The spectrum of , in combination with electronic structure calculations, suggests that the true ground state of is a ring structure with a transannular C–C bond, addressing a longstanding controversy surrounding this cluster. All three spectra exhibit Franck-Condon forbidden transitions; these are attributed to Herzberg-Teller coupling in and and autodetachment from an excited electronic state of .

Vibrational fine structure of C5 via anion slow photoelectron velocity-map imaging

High-resolution anion photoelectron spectra of cryogenically cooled C5(-) clusters are reported using slow photoelectron velocity-map imaging spectroscopy. We resolve vibronic transitions to the ν2 stretching mode and multiply excited ν5, ν6, and ν7 bending modes of neutral C5 with significantly higher accuracy than previous experiments. Weak transitions to Franck-Condon (FC) forbidden singly excited bending modes are made possible by Herzberg-Teller coupling between electronic states of the neutral cluster. In addition, we resolve vibrational fine structure corresponding to different angular momentum states of multiply excited bending modes. The observation of this multiplet structure, some of which is FC forbidden, is attributed to Renner-Teller coupling between vibrational levels in the C5(-) ground electronic state.

ZEKE spectroscopy with coherent vacuum ultraviolet radiation: The X 2∑g+ and A 2∏u states of N2+ in the 15.5 eV to 17.7 eV photon energy range

The zero kinetic energy (ZEKE) photoelectron spectrum of nitrogen has been recorded using coherent vacuum ultraviolet light for single-photon excitation to the various ionization thresholds. Ionization thresholds from the lowest (15.58 eV) to 17.7 eV were studied. In this energy range, both Franck–Condon allowed (v+=0 and 1) and Franck–Condon forbidden (v+=4, 5, 6, and 8) levels of the N2+ X 2∑g+ ground electronic state were studied. The rotational intensities of these bands were studied, and a comparison made with previous ab initio work on the photoionization of N2. The role of complex resonances in the alteration of rotational linestrengths is discussed with reference to a high resolution photoionization spectrum recorded in the region of the v+=1 threshold. It was also possible to compare the relative intensities of all of these vibrational bands, to investigate the mechanism for observation of Franck–Condon forbidden vibrational bands in ZEKE spectroscopy. These results are discussed in terms of resonant autoionization, which is commonly observed in threshold photoelectron spectroscopy. The v+=0, 1, 2, and 4 levels of the A 2∏u excited state of N2+ were also studied, and the rotational structure was compared with ab initio and experimental results for the same state in the isoelectronic system, CO+. The band intensities were also measured, and compared with the Franck–Condon factors for excitation to the A state vibrational levels for the v=0 state of N2. A comparison with the v+=0 band of the X state is used to obtain an estimate of the relative signal yield for these excited state thresholds. For most of the bands observed in the present study, there are previous lower resolution threshold photoelectron spectra recorded using synchrotron radiation, and a comparison is made with these previous results.

Intensities of forbidden pure torsional bands in S1–S spectra of toluenes

In toluenelike molecules with sixfold internal rotation symmetry, three weak, Franck–Condon forbidden, pure torsional transitions m21, m30, and m41 invariably appear in S1–S0 absorption spectra. The intensities are typically 1%–5% of the allowed bands, m00 and m11. Determination of the preferred conformer in S1 as staggered or eclipsed relies on proper assignment of the upper state of the m30 band as 3a″1 or 3a″2 under the molecular symmetry group G12. In addition, inferences of the preferred conformer in S0 and D0 (ground state cation) from band intensities of fluorescence and threshold photoionization spectra have also relied on the same assignment. For a set of six molecules having sixfold rotor potentials, including both –CH3 and –CD3 rotors, we present experimental relative intensities for the three forbidden S1–S0 absorption bands. Within an adiabatic electronic representation, we show how a Fourier expansion of the three components of the S1–S0 electric dipole transition moment predicts relative forbidden band intensities in quantitative agreement with experiment. This fixes the assignment of the upper state of the key m30 transition as 3a″1, establishes the preferred S1 conformer as staggered for all six molecules, and places earlier inferences of the preferred conformers in S0 and D0 on solid ground.

Rotationally resolved threshold photoelectron spectroscopy of O2 using coherent XUV: formation of vibrationally excited ions in the Franck–Condon gap

The rotationally resolved photoelectron spectrum of high vibrational levels of O2+ in the Franck–Condon gap was investigated using pulsed field ionization, zero kinetic energy photoelectron spectroscopy. By using a coherent extreme ultraviolet light source for single-photon excitation, the ν+ = 6–24 vibrational levels of O2+X2Πg were studied. This is the first time levels higher than ν+ = 14 have been observed with rotational resolution. The highest level studied in the present work had a vibrational energy corresponding to 70% of the well depth. Along with the novelty of the spectroscopic technique, the present results reveal interesting and new ionization dynamics. All levels observed are Franck–Condon forbidden, and are not observed in a conventional photoelectron spectrum. There was no direct relation between the vibrational bands and the autoionizing states observed in the photoionization efficiency spectrum in the same energy region, and the rotational line intensities observed in the various bands in the current threshold photoelectron spectra were all similar. The mechanism of this process was different from the "resonant autoionization" model, which involves coupling between a Rydberg state with a low principal quantum number, a dissociative state, arid the Rydberg series converging to the vibrationally excited ion states. Instead, the excitation process was believed to be more direct, involving mainly a dissociative state (or states) and the Rydberg states with a vibrationally excited ion core. Further investigation of this mechanism is still necessary, but the formation of these highly vibrationally excited ions opens a new horizon in state-selective reaction dynamics. With the coherent XUV light source and the PFI-ZEKE technique, a wide range of vibrational energies (up to 4 eV) can be deposited into the O2+ reactant with rovibrational selectivity.

Al+O3 chemiluminescence: Perturbations and vibrational population anomalies in the B 2Σ+ state of AlO

Ozone reacts with aluminum entrained in an argon buffer gas to yield the B 2Σ+–X 2Σ+ spectrum of AlO. The measured B 2Σ+ vibrational populations for this reaction follow a markedly non-Boltzmann distribution, exhibiting local maxima at vibrational levels v′=6, 8, 12, and 14. This behavior is attributed to an initial chemical reaction Al+O3→AlO(A 2Π) +O2 followed by the collision induced rearrangement AlO(A 2Π) +Ar→AlO(B 2Σ+)+Ar. The spin–orbit interaction in AlO connects ro-vibronic levels of the A 2Π and B 2Σ+ states. Consequently, collisional energy transfer is particularly efficient for the most strongly perturbed levels of the B 2Σ+ state. For Al+N2O, an approximate Boltzmann distribution in the AlO B 2Σ+ state population indicates that this reaction proceeds by a differing mechanism. Consistent with the proposed mechanism for Al+O3 is the appearance of ’’extra’’ band heads representing normally Franck–Condon forbidden A–X transitions which become allowed because of a small admixture of B 2Σ+ character. The relative intensities of the extra and ’’main’’ transitions are strongly dependent upon the argon buffer gas pressure. A quantitative description of this dependence gives an estimate for the amount of mixing between A 2Π and B 2Σ+ and for the rate of energy transfer between these two states. The measured rate constant for the most strongly perturbed level is comparable in magnitude to the rate constant expected for hard sphere collisions between argon and AlO. The present results allow some understanding of the continuum emission observed in the upper atmosphere release of aluminized grenades. They also may be pertinent to the construction of a visible chemical laser.

Catalysis of "Franck-Condon forbidden" electron-transfer reactions by macrocyclic cobalt(II) complexes

Catalysis of "Franck-Condon forbidden" electron-transfer reactions by macrocyclic cobalt(II) complexes