Picosecond time-resolved nonresonant ionization detected IR spectroscopy on 7-azaindole dimer

The picosecond time-resolved IR spectrum of the 7-azaindole dimer has been measured by picosecond time-resolved nonresonant ionization detected IR spectroscopy. This new time-resolved technique was developed by combining nonresonant ionization detected IR (NID-IR) spectroscopy with tunable picosecond IR and UV lasers. The time-resolved NID-IR spectrum from 2 600 cm^-1 to 3 800 cm^-1 shows a drastic change from 1.5 ps to 11 ps time evolution. A mode-specific vibrational redistribution has been suggested. Received 14 May 2002 / Received in final form 11 June 2002 Published online 13 September 2002     

Water dimer vibration-rotation tunnelling levels from vibrationally averaged monomer wavefunctions

The vibration-rotation tunnelling (VRT) spectra for the water dimer obtained by vibrationally averaging the dimer potential over accurate water monomer wavefunctions is reported. The vibrational averaging requires evaluation of the 12D dimer potential energy surface at more than 10¹² distinct geometries. The resulting vibrational spectra of the low-lying dimer states are presented and compared with both less computationally expensive methods based on fixed nuclei approximations and the recent (6+6)d adiabatic calculations of Leforestier et al. [38] (2009). The procedure gives some modest improvement in the agreement with experimental values for the vibration-rotation tunnelling (VRT) states of (H₂O)₂ and (D₂O)₂. This approach can be extended to treat dimer states involving monomer overtone excitations, which is important in obtaining water dimer spectra at infrared and visible wavelengths at atmospheric temperatures, and in characterizing the dimer contribution to the so-called water continuum absorption at these wavelengths.     

Lower excited states of 7-azaindole dimer with a C2h(M) symmetry

The title subjects (in the vapor phase) were assessed to be in the weak-coupling limit as Frenkel-type excitons of a L_a-type excited state, by examining reported data on these systems: (1) L_b/L_a interconversion by dimerization (in the solution) just like what happens in the monomer by the change of solvent polarity, e.g., from 3-methylpentane to ethanol. (2) Good correspondence between the monophotonic excitation spectrum of the tautomer fluorescence (after excited-state double proton-transfer reaction of the dimer) and mass-selected (2 + 2) photoionization spectrum of the dimer, as is predicted theoretically. (3) Potential minima for locally excited configurations, as were predicted by ab initio calculations. Comments on the current controversy about the mechanism of its excited-state tautomerization, i.e., whether it is of one step or two steps, are made as well.     

Determination of the excited state structure of 7-azaindole using a Franck-Condon analysis

The change in structure of 7-azaindole upon electronic excitation was determined by a Franck-Condon analysis of the intensities in the fluorescence emission spectra obtained via excitation of six different vibronic bands. A total of 107 emission band intensities were fit, together with the changes in the rotational constants of four 7-azaindole isotopomers. The geometry change of the ring framework upon electronic excitation from the electronic ground state to the ¹L_b state (ππ*) can be described by an overall expansion of the pyridine ring of 7-azaindole, with minor changes of the pyrrole ring. The resulting geometry changes are interpreted on the basis of ab initio calculations.     

Spectroscopy of doubly hydrogen-bonded 7-azaindole. Reinvestigation of the excited state reaction

Luminescence of 7-azaindole (7AI) in non-polar solvents and alcohols was investigated with respect to its dependence upon temperature, concentration, and excitation energy. It was found that at low temperature oligomeric structures of 7AI are formed in the ground state. High-energy fluorescence (F₁) was shown to consist of dimeric and oligomeric contributions. The phosphorescence was assigned to oligomers. The mechanism of excited state cooperative double proton transfer is discussed along with the results of INDO/S calculations of transition energies and charge densities.     

New Insights on the 7-azaindole Photophysics: The Overlooked Role of Its Non Phototautomerizable Hydrogen Bonded Complexes

In this paper we explore the formation and the photophysical properties of the scarcely studied open hydrogen bonded aggregates of 7-Azaindole, 7AI. Thus, we have analyzed the influence that the increase of the 7AI concentration and the decrease of the temperature have on the 7AI photophysics. To help the interpretation of the results, the 7AI-Pyridine system has been used as the model for the analysis of the photophysical properties attributable to the open N_pyrrolic ? HN_pyridinic hydrogen bonded aggregates. Also, the hydrogen bond interactions have been studied by means of the atom in molecule approach from the Bader theory. Experimental and theoretical results support that the formation of open hydrogen bonded aggregates, (?7AI-)_n with n?≥?2 can efficiently compete with that of the profusely studied centro-symmetric cyclic dimer (7AI)₂. Moreover, these aggregates suffer a proton-driven electron transfer process that strongly quenches their room temperature fluorescence and, therefore, masks their presence in the 7AI solutions. Therefore, because most of the studies on the 7AI photophysics have been interpreted without considering the existence of such aggregates and, more important, ignoring its quenching process, many conclusions obtained from these studies should be carefully revised.     

Phonon-assisted tunnelling in the electroluminescence exciting process

Experimental results on electroluminescence brightness dependence on temperature and applied voltage for zinc sulphide luminophors are fitted to electron tunnelling rate dependence on temperature and field strength, computed according to the recent phonon-assisted tunnelling theory. A satisfactory agreement of experimental data with theory is observed. It is concluded that the processes of free carrier generation by an electric field play a decisive role in these dependences. On the basis of this conception, the analysis of brightness dependence on temperature and applied voltage allows the determination of the exciting field strength in barriers of the energy of the phonons taking part in the electron tunnelling process.     

Resonant tunnelling lifetime in the semiconductor superlattice

The fundamental physics of the resonant tunnelling lifetime (RTL) in superlattices have been theoretically studied. The modelling of this RTL is based on a relationship between the resonant tunnelling and the half-width at half-maximum of the transmission peak. The lifetime of resonant states and the current density accompany the resonant tunnelling change as a function of mole fraction of the barrier layer, well width and barrier width. The energies and the lifetime of the electrons at the resonant states are correlated with the band structure of the superlattices. It is seen that the variation in RTL with resonance energy has a special minima, and that these minima occur around the centre of the allowed bands.     

Spin tunnelling in mesoscopic systems

We study spin tunnelling in molecular magnets as an instance of a mesoscopic phenomenon, with special emphasis on the molecule Fe₈. We show that the tunnel splitting between various pairs of Zeeman levels in this molecule oscillates as a function of applied magnetic field, vanishing completely at special points in the space of magnetic fields, known as diabolical points. This phenomena is explained in terms of two approaches, one based on spin-coherent-state path integrals, and the other on a generalization of the phase integral (or WKB) method to difference equations. Explicit formulas for the diabolical points are obtained for a model Hamiltonian.     

Energy loss in quantum tunnelling

The energy dissipated during the quantum decay of a metastable system which interacts with an environment at zero temperature is considered. It is found that quantum effects reduce the energy loss as compared to simple semiclassical estimates and that the decay is always possible for energetical reasons. Explicit results for the decay rate are obtained from the functional integral of the partition function.     

Resonant tunnelling in O-D systems

O-D energy spectroscopy by means of transport and magneto-transport measurements has been carried out in double barrier resonant tunnelling heterostructures. Two models for the fabrication imposed lateral confining potential have been considered to account for the resonant lines associated with tunnelling of electrons through the O-D states of the quantum well. Preliminary measurements in a magnetic field are consistent with the values of magnetic length and undepleted conducting core radius in our structure, and indicate that up to 6 T the energy spectrum is dominated by spatial quantization. Single-charging effects in our structures are discussed.     

Time of scattering in the one-dimensional inelastic tunnelling

Summary The various definitions for time of tunnelling across a barrier in one dimension, that have been introduced in the elastic case and discussed by Buttiker and various authors, are here extended allowing for inelastic losses to occur inside the barrier. The framework of a multichannel scattering theory is here used. Their relevance to inelastic tunnelling and resonant tunnelling is critically analysed. Results are reported for the average delay time of eq. (24) in the case of an inelastic point interaction with a harmonic oscillator inside the barrier. To speed up publication, the author of this paper has agreed to not receive the proofs for correction.     

On the theory of electron tunnelling

A generalized expression for the differntial tunnelling current density based on the problem of electron energy distribution introduced during the process has been presented. This is directly applicable to junction devices for the evaluation of their tunnelling I–V characteristics. A list of symbols appear at the end of the paper.     

Role of friction in tunnelling motion

One of the most interesting recent topics in condensed matter physics concerns the quantum tunnelling phenomena at low temperatures in the presence of dissipation. Such studies can be made of the motion of light interstitials (μ⁺ or H, for instance) in metals, with the aid of muon spin rotation and neutron scattering techniques. In this paper, the present status of the subject will be reviewed. Next, the utility of Mossbauer spectroscopy in analyzing a similar situation, viz. the motion of a reaction coordinate coupled to a dissipative bath, as in biological molecules, will be indicated.     

Trap-assisted tunnelling current in MIM structures

A new model is presented of current transport in Metal Insulator Metal (MIM) structures by quantum mechanical tunnelling. In addition to direct tunnelling through an insulating layer, tunnelling via defects present in the insulating layer plays an important role. Examples of the influence of the material and thickness of the insulating layer, energy distribution of traps, and metal work functions are also provided.     

Optical-phonon tunnelling and the electron scattering rate

Tunnelling of optical phonons across a quantum well is possible for a polar mode provided its frequency coincides with that of the two interface modes associated with the barrier. Because of this effect LO modes could scatter electrons in the well. In this paper we report a calculation of the total scattering rate caused by modes in the LO branch of the barrier material as a function of well width. We show that the form of the well-width dependence and the magnitude of the scattering rate is, to an excellent approximation, the same as that predicted by the dielectric continuum (DC) model, in which the scattering is caused by barrier interface modes obeying only electrical boundary conditions. This result provides, for the first time, justification for using Fuchs–Kliewer-like interface modes, even though these do not satisfy the necessary mechanical boundary conditions at the interfaces. It completes a study of the applicability of the DC model to the calculation of the scattering rates in a quantum well, a study that has already shown it to give a good agreement with the results of the hybrid model as regards the effects of modes in the well are concerned [12]. Our conclusion is that provided that the frequency-dependence of the coupling strength of the interface modes is correctly described, differences in detail concerning the effect of individual modes in the two models are unimportant for the total sum.     

On the measurement of electron tunnelling time

A suggestion to experimentally measure the electron tunnelling time by observing the tunnelling current cut-off as a function of the magnetic field intensity in semiconductor pn tunnel junctions, when they are placed in a crossed electric and magnetic field configuration, has been made in this paper. A simple and a rigorous quantum mechanical analysis to justify the above proposition have been presented. An order of agreement between the tunnelling time values derived from the published experimental data and our theoretical prediction has been noticed.