H. Bitto

Bio: H. Bitto is an academic researcher from University of Zurich. The author has contributed to research in topics: Quantum beats & Hyperfine structure. The author has an hindex of 13, co-authored 37 publications receiving 523 citations.

Hyperfine quantum beats and Zeeman spectroscopy in the polyatomic molecule propynal HC≡CCHO

Abstract: Quantum beats in the fluorescence decay of two single rovibronic levels of optically excited propynal (HC≡CCHO) have been measured in a supersonic jet apparatus. The complex beat pattern was explored using laser light of linear and circular polarization under zero‐field conditions (nulled earth magnetic field) or in conjunction with an applied magnetic field. Theoretical aspects of molecular quantum beats were derived which include hyperfine coupling, magnetic field effects, and polarized excitation and detection. Singlet–triplet coupling matrix elements, Lande g‐factors of the triplet hyperfine levels, and singlet and triplet decay rates have been determined.

Molecular quantum beat spectroscopy

Abstract: The present account of high-resolution molecular quantum beat spectroscopy emphasizes the versatility of this powerful Doppler-free method. Illustrations are drawn from work on the six-atomic molecule propynal carried out in the authors' laboratory. Examples of anisotropic Zeeman, Stark and hyperfine quantum beats, of isotropic molecular quantum beats and of multi-level quantum beats are discussed with regard to intramolecular relaxation dynamics and the determination of structural parameters such as nuclear hyperfine constants, electric dipole moments, spin-orbit matrix elements and asymmetry-splittings.

Quantum beats in the S1 dynamics of acetaldehyde

Abstract: We have investigated jet-cooled acetaldehyde, CH 3 CHO and CD 3 CDO, excited to the S 1 (nπ ∗ ) singlet state. For excess energies Δ E (S 1 ) = 1500–2000 cm −1 , we observed fluorescence decays with superimposed quantum beats due to coherently excited S 1 –T 1 eigenstates and an increase in the decay rates parallelled by a decrease in the fluorescence, most dramatic at Δ E (S 1 ) ≈ 2000 cm −1 . These results show that S 1 –T 1 intersystem crossing is not a dissipative process up to the fluorescence breakdown. It is proposed that at this energy the continuum for α-cleavage in the T 1 state is reached which causes the S ⇝ T 1 intersystem crossing to become the dominant deactivation pathway.

Nuclear hyperfine structure in the electronic spectrum of propynal

Abstract: The laser‐induced fluorescence spectrum of propynal (S1←S0) (A 1A‘←X 1A’) has been investigated and a series of perturbations due to singlet–triplet coupling resolved. The singlet–triplet interaction matrix elements are of the order of 280 MHz. Fluorescence decays for both the mixed singlet–triplet eigenstates have been measured and are shown to contain quantum beats due to coherent excitation of nuclear hyperfine levels. This Doppler‐free measurement in the time domain leads to an accurate determination of the nuclear hyperfine splittings produced by the two nonequivalent hydrogen atoms. The hyperfine structure has been analyzed for two rotational levels in the 91 vibrational state and gives the following values for combinations of the Fermi contact constants and dipole–dipole constants of the two protons in the pure triplet state: T1(a 3A‘): A(1)FC+ 1/2 (T(1)xx−T(1)yy) ∼46 MHz; A(2)FC+ 1/2 (T(2)xx−T(2)yy) −4 MHz; T(1)zz∼T(2)zz ∼0 MHz. A simple model for the spin distribution in propynal predicts that...

Vibrational and rotational level dependence of the S1 decay of propynal in a supersonic jet

Abstract: Single vibronic level and single rovibronic level decays of the first excited singlet state [ΔEvib (S1)≤1300 cm−1] of propynal (HCCCHO) and propynal‐d1 (HCCCDO) have been investigated in a pulsed supersonic jet. For this purpose, the emission decay rates were measured for ∼200 rotational levels in 14 vibronic bands of HCCCHO and ∼100 rotational levels in nine vibronic states of HCCCDO. The internal conversion process, which is the main deactivation route of the S1 state in HCCCHO, was found distinctly mode dependent with the ν10 vibration (CHald wagging mode) acting as the dominant promoting mode. The nonradiative decay rates showed no simple systematic dependence of the rotational quantum numbers J and K. For a number of rotational levels an oscillatory decay behavior (quantum beats) was observed the analysis of which provided information on the S1–T1 coupling in propynal.

The spectroscopy of solvation in hydrogen-bonded aromatic clusters

Abstract: ▪ Abstract Various aspects of molecular solvation are reviewed from the perspective provided by gas-phase aromatic solute-(solvent)n clusters. Particular emphasis is placed on hydrogen-bonded clusters, varying from 1:1 aromatic-H2O complexes up to clusters containing several water or methanol molecules. Recent advances in experimental methods for obtaining accurate structures, binding energies, and intermolecular and intramolecular vibrational spectra are highlighted. Many of these methods provide size and conformation selectivity and can be readily extended to both ground and electronically excited neutral states. The π hydrogen bond, hydrogen bonding to aromatic alcohols, water complexation to indole and its derivatives, and the hydrogen-bonded networks of benzene-(H2O)n and benzene-(CH3OH)n clusters are reviewed in special detail.

Purely rotational coherence effect and time‐resolved sub‐Doppler spectroscopy of large molecules. II. Experimental

Abstract: In this and the accompanying paper we present a theoretical treatment and experimental study, respectively, of the phenomenon termed purely rotational coherence. This phenomenon has been demonstrated to be useful as a time domain means by which to obtain high resolution spectroscopic information on excited state rotational levels of large molecules [Felker et al., J. Phys. Chem. 90, 724 (1986); Baskin et al., J. Chem. Phys. 84, 4708 (1986)]. Here, the manifestations in temporally resolved, polarization-analyzed fluorescence of coherently prepared rotational levels in samples of isolated symmetric and asymmetric top molecules are considered. These manifestations, for reasonably large molecules at rotational temperatures characteristic of jet-cooled samples, take the form of polarization-dependent transients and recurrences with temporal widths of the order of tens of picoseconds or less. The transients, which arise from the thermal averaging of many single molecule coherences, are examined with respect to their dependences on molecular parameters (rotational constants, transition dipole directions) and experimental parameters (polarization directions and temperature). A physical picture of rotational coherence as a reflection of the time-dependent orientation of molecules in the sample is developed. And, the influence of rotational coherence in experiments designed to probe intramolecular energy flow is discussed. In the accompanying paper, we present experimental results for jet-cooled t-stilbene and anthracene. For t-stilbene we determine rotational constants for vibrational levels in the S1 electronic state (from the recurrences) and we monitor the trends in rotational coherence vs vibrational coherence as the total energy in the molecule increases.

Sub‐Doppler laser‐induced fluorescence measurements of the velocity distribution and rotational alignment of NO photofragments

Abstract: The photodissociation of dimethylnitrosamine, (CH3)2N–NO, at 363.5 nm produces ro‐vibrationally excited NO fragments. With two‐photon laser‐induced fluorescence the Doppler profiles of single rotational transitions were probed, using various polarization schemes for dissociation and probe lasers. These measurements provided a detailed product state distribution, the three‐dimensional recoil velocity distribution of specific fragment states, and the alignment of their angular momentum. We present evidence of the presence of correlations between fragment recoil direction and alignment of fragment angular momentum.

Nonlinear refraction in CS 2

Abstract: The nonlinear refractive index (γ) of CS2 was measured using the Z-scan technique and laser radiation of various (femto-, pico-, and nano-second) pulse durations. We observed the growth of γ with the increase of the pulse duration (from (3±0.6)×10-15 cm2 W-1 at 110 fs to (4±2)×10-14 cm2 W-1 at 75 ns) due to the additional influence of the molecular reorientational Kerr effect in the case of longer (picosecond and nanosecond) pulses. Acoustic wave induced negative nonlinear refraction was observed using wavefront analysis. We analyzed the simultaneous influence of both electronic and molecular processes leading to the positive nonlinear refraction and acoustic processes leading to the negative nonlinear refraction in carbon disulfide. Variations of the refractive index due to the thermal effect at high pulse repetition rates were also investigated.