Niloufar Shafizadeh

Photodissociation of H2O and D2O below 132 nm

A complete determination of the rotational and vibrational distributions in the OH/OD(A 2Σ+) fragments which result from the VUV photodissociation of H2O/D2O as a function of excitation energy is presented. VUV excitation was performed at eight different wavelengths (eleven for D2O) in the range between 132 and 124 nm. The vibrational branching ratios show a relatively fast rise of the vibrational excitation at threshold followed by a plateau. The general trend of the experimental results is well reproduced by a phase space theory calculation and by ab initio calculations reported by van Hemert and van Harrevelt. Rotational distributions seem to depend only on the available excess energy and, for a given excess energy appear to be similar for all vibrational levels in OH and OD.

A,C, and D electronic states of the Ar–NO van der Waals molecule revisited: Experiment and theory

The A–X transition of ArNO has been reinvestigated by laser induced fluorescence (LIF) both in the bound–free and bound–bound region. The discrete part of the spectrum is at least two orders of magnitude weaker than the continuum part, indicative of a large change in geometry from the ground state. This very different configuration, both from the ground state and from the C and D states, can only be explained by strong interactions, induced by the perturbing argon atom, between the excited states of the van der Waals complex converging to the 3sσ,A, 3pπ,C, and 3pσ,D Rydberg states of NO. In order to quantitatively understand the observed structure of the A–X, C–X, and D–X excitation spectra, a global theoretical approach is proposed, based on ab initio calculations of the potential energy surfaces in the planar A′ and A″ symmetries, including a configuration interaction between the states of same symmetry. Small adjustments of the diabatic energy surfaces lead to a satisfactory agreement between the obser...

nf Rydberg complexes of NO in a magnetic field, probed by double resonance multiphoton ionization

nf (v=1) Rydberg states of NO have been probed by double resonance multiphoton ionization in a 1 T external magnetic field. Due to the nonpenetrating character of the f orbitals, these Rydberg states are very sensitive probes of any external perturbation. As n increases, a decoupling of the angular momentum l of the Rydberg electron from the molecular frame occurs gradually, as the magnetic interaction becomes more and more important with respect to intramolecular forces. Up to n≂15, only the linear Zeeman perturbation has been taken into account. The rotational–electronic structure of the 7f and 15f states has been interpreted theoretically by considering the linear Zeeman perturbation in addition to the Coulombic interaction and the long range interaction due to the quadrupole moment and the polarizability of the ion core. The intensities and line positions of the transitions from the intermediate A 2Σ+,v=1 level to the 7f and 15f levels have been calculated. The alignment of the N, MS, MN Zeeman sublev...

Photodissociation of H2O in the “second continuum”

Abstract The photodissociation of H2O near the threshold for production of excited OH(A 2Σ+) fragments has been studied by two-photon excitation of the B 1A1 state at 266 nm. The rotational population distribution is found to peak between N′ = 6 and 9 with a strong contribution in the N′=2 to 5 range. The long-lived “trapped” trajectories proposed by Segev and Shapiro are not positively confirmed by this test but cannot be excluded either.

First observation in the gas phase of the ultrafast electronic relaxation pathways of the S2 states of heme and hemin

The time evolution of electronically excited heme (iron II protoporphyrin IX, [Fe(II) PP]) and its associated salt hemin (iron III protoporphyrin IX chloride, [Fe(III) PP-Cl]), has been investigated for the first time in the gas phase by femtosecond pump-probe spectroscopy. The porphyrins were excited at 400 nm in the S(2) state (Soret band) and their relaxation dynamics was probed by multiphoton ionization at 800 nm. This time evolution was compared with that of the excited state of zinc protoporphyrin IX [Zn PP] whose S(2) excited state likely decays to the long lived S(1) state through a conical intersection, in less than 100 fs. Instead, for [Fe(II) PP] and [Fe(III) PP-Cl], the key relaxation step from S(2) is interpreted as an ultrafast charge transfer from the porphyrin excited orbital π* to a vacant d orbital on the iron atom (ligand to metal charge transfer, LMCT). This intermediate LMCT state then relaxes to the ground state within 250 fs. Through this work a new, serendipitous, preparation step was found for Fe(II) porphyrins, in the gas phase.

An Efficient Indirect Mechanism for the Ultrafast Intersystem Crossing in Copper Porphyrins

The ultrafast dynamics of copper tetraphenylporphyrin (CuTPP), copper octaethylporphyrin (CuOEP), and of the free base tetraphenylporphyrin (H2TPP), excited in the S2 state have been investigated in the gas phase by femtosecond pump/probe experiments. The porphyrins were excited in the Soret band at 400 nm. Strikingly, the S2-S1 internal conversion in H2TPP is very rapid (110 fs), as compared to that of ZnTPP (600 fs), previously observed. In turn, CuTPP and CuOEP, excited in S2, follow an efficient and different relaxation pathway from that of other open-shell metalloporphyrins. These two molecules exhibit a sequential four-step decay ending on a slow evolution in the nanosecond range (2)S2 → (2)CT → (2)T → (2)Ground State. This latter evolution is linked to the formation of the (2)T, tripdoublet state in CuTPP, observed in the condensed phase. It is shown that an intermediate charge transfer state plays a crucial role in linking the porphyrin centered (1)ππ* and (3)ππ* configurations. A simple model is presented that allows a rapid evolution between these two configurations, via coupling of the porphyrin π system with the free d electron on the copper. The mechanism obviates the need for the spin orbit coupling within the porphyrin. The result is that these copper porphyrins can exhibit an ultrafast apparent intersystem crossing, unprecedented for organic molecules.

Visible emission from the vibrationally hot C2H radical following vacuum-ultraviolet photolysis of acetylene: Experiment and theory

Photolysis of acetylene has been performed by vacuum-ultraviolet excitation with the synchrotron radiation via the Rydberg states converging to the first ionization potential (IP) at 11.4 eV. Only the visible fluorescence of the ethynyl radical was observed in the A 2Π–X 2Σ+ system. Excitation of several Rydberg states of acetylene over a large energy range between 9 and 11.4 eV allowed us to observe for the first time the evolution of this continuum with increasing Rydberg excitation. Intensity calculations based on accurate ab initio potential energy surfaces of C2H were performed by using a one-dimensional model accounting for the large-amplitude motion of the H atom around the C–C bond and for the overall rotation of the radical. These calculations successfully reproduce the observed visible continuum (maximum at 500 nm and blue side cutoff at 400 nm) and bring new information on the distribution of the internal energy deposited in the fragment. For most excited Rydberg states, predissociation occurs...

Observation of the lowest 1Δu Rydberg state of acetylene by multiphoton ionization spectroscopy

The 1Δu, 3dπ component of the lowest ungerade 3d+4s Rydberg supercomplex of acetylene has been observed by (3+1) photon ionization spectroscopy of C2H2 and C2D2. In both isotopic species the vibrationless level of this new electronic state is sufficiently long lived to support rotational structure. From the observed isotopic shift and from the rotational and vibrational parameters determined in the present analyses, this state could be unambiguously identified as the lowest Rydberg state of 1Δu symmetry, belonging to the 3d complex of acetylene. The weak intensity of this three‐photon 1Δu–1Σ+g electronic transition is discussed in comparison with the other Rydberg transitions within the same supercomplex. Excited vibronic bands in the same energy region allowed to determine the ν1 and ν2 frequencies for some components of this Rydberg supercomplex. The absence of the nearby H 1Πu, 3dδ Rydberg state in the MPI spectra is discussed in terms of predissociation.

The a 2Π-X2Σ+ system of MgCl. Evidence for predissociation in the a 2Π state of MgCl

Abstract The A 2Π-X2Σ+ system of MgCl has been reinvestigated. Laser excitation spectra of MgCl formed in a Broida oven, from the reaction Mg (3P) +R-Cl (R = Cl, CH2Cl, CHCl2, CCl3, C2H4Cl, C4H8Cl, C6H5, and CH2C6H5) have been recorded under medium resolution. The sudden cut-off of the vibrational structure (at VA⪰7), in all cases, is interpreted as arising from an avoided crossing between the … (3π)4 (4π)1 regular A 2II state and the … (3π)3 repulsive and inverted (D)2Π state. The strong interaction between the two 2Π states results in a predissociation of the A state and would explain the observed anomalous r-dependence of ΔGv +1 2 and Avso. An overall fit of all the presently available spectroscopic data provided a consistent set of vibrational and rotational constants for the A and X states of MgCl.

Observation of an excimer emission band of the ArOD van der Waals molecule

An excimer emission band attributed to a transition from the first excited state to the ground state of the van der Waals Ar(OD) molecule has been observed in experiments where D2O was dissociated in presence of rare gases by two photons of a quadrupled YAG laser (266 nm). KrOD has also been observed in the same experiments. The ArOD excimer band comprises three main peaks at 313.9, 315.1, and 316.0 nm as well as several minor ones nearer to the main OH (A2 Σ+ ,v=0–X2 Π,v=0) emission band. The emission profile is realistically simulated by a one dimension model using recently published potentials for the excited state and modeling the ground state by that of the ArF X 2 Σ+ state. This result tends to demonstrate that the ground state geometry of Ar(OD) involved in the transition is probably L shaped although the high intensity of the observed excimer band is not satisfactorily explained.