P. Schmidt

Bio: P. Schmidt is an academic researcher from University of Zurich. The author has contributed to research in topics: Propynal & Hyperfine structure. The author has an hindex of 5, co-authored 7 publications receiving 160 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.

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...

Measurement of hyperfine polarization quantum beats in a polyatomic molecule

Abstract: Polarization quantum beats between hyperfine components of perturbed rotational states in the S1→S0 fluorescence of propynal (HC≡CCHO) have been measured. The quantum beats are explained by a periodic transfer of molecular alignment to nuclear spin polarization.

Excited state dipole moments in a polyatomic molecule determined by Stark quantum beat spectroscopy

Abstract: High resolution Stark quantum beat spectroscopy is demonstrated to be a powerful method to determine magnitude and orientation of excited state dipole moments in asymmetric polyatomic molecules. We measured the dipole moments μ of the vibrationless S1 state of planar propynal and αD‐propynal in a pulsed supersonic jet and determined both components of the μ vector in the molecular plane: μ’a=0.874(10)D, μb=1.06(2)D, ‖μ’‖=1.374(22) D for HC≡CCHO; μ’a=0.863(10)D, μb=1.04(1) D ‖μ’‖=1.351(14) D for HC≡CCDO. The results are related with recently reported microwave data of the ground state dipole moment μ(S0) and are interpreted in terms of electron delocalization from the carbonyl group to the ethinyl moiety. The dipole moments of S0 and S1 propynal are oriented essentially along the CO bond. The applicability of the Stark quantum beat technique to polyatomic molecules is discussed.

Determination of asymmetry splittings in the polyatomic molecule propynal by Stark quantum beat spectroscopy

Abstract: Coherences among asymmetry-split rotational levels in a molecule can be created when a weak electric field is applied. The quantum beats superimposed on the time-resolved fluorescence decay are utilized for the accurate measurement of asymmetry splittings. The technique is exemplified for selected vibronicS1 states of propynal and αD-propynal and the results are compared with conventional (i.e. frequency domain) spectroscopic data. The applicability of the presented method of coherence spectroscopy is discussed.

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.

Rovibronic level structure of electronically excited acetylene (A1Au) in a supersonic jet as studied by laser-induced fluorescence and Zeeman quantum beat spectroscopy

Abstract: Laser-induced fluorescence (LIF) spectra of the A 1Au- X 1Σg+ transition of jet-cooled acetylene were measured resolution to resolve the respective rovibronic levels in the A nv′3 vibrational states. Most of rotational levels of the nv′ and 4) state split into several levels, while level splitting occurs scarcely for those in the 2v′3 state. The observed radiative lifetimes of the respective levels are different from each other. Especially, the average lifetime of levels in the 3v′3 state is relatively long compared with those in the other states. The fluorescence from a number of levels decays in an oscillatory manner under a weak magnetic field. This phenomenon is attributed to a coherent interaction between the Zeeman sublevels, from which a magnetic moment, i.e., a g-factor, can be determined characteristic to each level. A large g-factor assigned to a level implies that the level couples strongly with a triplet state. Long lifetimes and large g-factors found for levels in the 3v′3 state imply that these levels couple with triplet states more efficiently than other levels in the 4v′3 or 2v′3 state. Besides the Zeeman quantum beat, a quantum beat ascribed to level anticrossing between a level in the A nv′3 state and a nonfluorescent level is found for a number of excited levels. These anticrossings are due to interaction with its strength of the order of MHz, and ascribed to couplings with vibrationally highly excited levels in the X state.

A study of the singlet–triplet perturbations in the à 1Au state of acetylene by high resolution ultraviolet spectroscopy

Abstract: Laser‐induced fluorescence spectra of the 330 K10 and 340 K10 vibronic bands of the A 1Au←X 1Σ+g transition in acetylene have been recorded with a resolution of 18 MHz. Each rotational transition consists of a group of lines due to coupling of the electronically excited singlet state with isoenergetic triplet states. Using the standard deconvolution procedure the singlet–triplet coupling elements and the density of coupled triplet states are derived for rotational levels up to J=4 in both bands. From the density of coupled triplet states it is concluded that the A 1Au state is perturbed by the T1 3B2 state. Magnetic field measurements have shown that the predissociation of acetylene in the 4ν3’ vibrational level of the A state is caused by a coupling via the T1 3B2 state with predissociating vibrational levels of the electronic ground state.

Quantum beat spectroscopy of molecules

Abstract: This review deals with molecular high-resolution spectroscopy based on the quantum beat phenomenon. Applications of the essentially Doppler-free quantum beat technique are presented with special emphasis on the theoretical description. After an historical introduction and a general discussion of interference experiments, we discuss the expressions to describe the time-resolved fluorescence of a molecule following well defined laser excitation. Our general treatment for the characterization of the excited superposition state enables us to demonstrate the versatility of the method by identifying the excited molecular eigenstates with, for example, Zeeman levels or fine- and hyperfine-structure levels. In addition to the spectroscopic applications of quantum beat experiments in determining molecular structure parameters, we address the applications to molecular dynamics and statistical properties of molecular level structures.