P. Decleva

Ultrafast Electron Dynamics in Phenylalanine Initiated by Attosecond Pulses

In the past decade, attosecond technology has opened up the investigation of ultrafast electronic processes in atoms, simple molecules, and solids. Here, we report the application of isolated attosecond pulses to prompt ionization of the amino acid phenylalanine and the subsequent detection of ultrafast dynamics on a sub–4.5-femtosecond temporal scale, which is shorter than the vibrational response of the molecule. The ability to initiate and observe such electronic dynamics in polyatomic molecules represents a crucial step forward in attosecond science, which is progressively moving toward the investigation of more and more complex systems. Electronic dynamics in a complex polyatomic molecule are tracked faster than the time scale for vibrational motion. A very quick look at phenylalanine Over the past decade, laser technology has pushed back the fastest directly observable time scale from femtoseconds (quadrillionths of a second) to attoseconds (quintillionths of a second). For the most part, attosecond studies so far have probed very simple molecules such as H2 and O2. Calegari et al. now look at a more elaborate molecule—the amino acid phenylalanine. They tracked changes in the electronic structure of the compound after absorption of an ultrafast pulse, before the onset of conventional vibrational motion. Science, this issue p. 336

Convergence of the multicenter B-spline DFT approach for the continuum

A multicenter approach for the calculation of the electronic continuum spectrum based on the B-spline functions and employing a Kohn–Sham density functional hamiltonian is introduced. The method is based on a large expansion on the origin, supplemented by a limited number of off-center functions located on the positions of the nuclei. The method has been applied to study the photoionization of Cl2, of the model system ðCOÞ 2 and of CrðCOÞ 6 . The method has proven very efficient: the convergence to the exact results is obtained with matrices which are much smaller than those involved in one center expansion calculations and the algorithm is numerically stable up to very high photoelectron energies (200 eV). The importance of the asymptotic moment basis set requirement is pointed out and rationalized with the help of a simple model. Preliminary calculations on CrðCOÞ 6 are then presented and their convergence discussed. 2002 Elsevier Science B.V. All rights reserved.

Valence photoionization dynamics in circular dichroism of chiral free molecules: The methyl-oxirane

The dynamical behavior of circular dichroism for valence photoionization processes in pure enantiomers of randomly oriented methyl-oxirane molecules has been studied by circularly polarized synchrotron radiation. Experimental results of the dichroism coefficient obtained for valence photoionization processes as a function of photon energy have been compared with theoretical values predicted by state-of-the-art ab initio density-functional theory. The circular dichroism measured at low electron kinetic energies was as large as 11%. Trends in the experimental dynamical behavior of the dichroism coefficients D(i)(omega) have been observed. Agreement between experimental and theoretical results permits unambiguous identification of the enantiomer and of the individual orbitals.

Variational approach to continuum orbitals in a spline basis: An application to H2+ photoionization

Abstract A variational approach for the determination of continuum orbitals, recently proposed in the one-dimensional case, is generalized to the multicenter molecular problem. A test application is performed on the H2+ molecule in the one-center approximation, with a radial basis or B-spline functions. The resulting eigenvalue equation is solved by inverse subspace iteration giving a full set of K-matrix normalized solutions at each prefixed energy. Excellent numerical stability allows expansion up to l=20, giving very satisfactory agreement with exact results. The potential of a multicenter LCAO-type implementation is pointed out.

Time-dependent density-functional theory for molecular photoionization with noniterative algorithm and multicenter B-spline basis set: CS2 and C6H6 case studies

In this work a new direct (noniterative) algorithm to solve the time-dependent density-functional theory equations for molecular photoionization has been proposed and implemented, using a multicentric basis set expansion of B-spline functions and complete exploiting of the molecular point-group symmetry. The method has been applied to study the photoionization dynamics of CS2 and C6H6: the results confirmed the expectation of large screening effects in CS2. For C6H6 the screening effects have been found to play a minor role than in CS2, however, also in this case the quality of the final results is definitely improved. The method has proven suitable to study with confidence molecules of medium size, and there is still room for further improvement working on more elaborate treatment of the exchange-correlation functional.

Density functional study on the circular dichroism of photoelectron angular distribution from chiral derivatives of oxirane

The linear combination of atomic orbitals B-spline density functional method has been successfully applied to a series of four chiral derivatives of oxirane, to calculate the photoionization dynamical parameters, the circular dichroism in the angular distribution effect, and to identify trends along the series. The computational algorithm has proven numerically stable and computationally competitive. The photoionization cross section, asymmetry, and dichroic parameter profiles relative to valence orbitals have been systematically studied for the states which retain their nature along the series: the identified trends have been ascribed to the different electronic properties of the substituents. A rather unexpected sensitivity of the dichroic parameter to changes in the electronic structure has been found in many instances, making this dynamical property suitable to investigate the electronic structure of chiral compounds. The magnitude of the circular dichroism in the angular distribution effect does not seem to be associated with the initial state chirality, but rather to be governed by the ability of the delocalized photoelectron wave function to probe the asymmetry of the molecular effective potential.

Time-dependent density functional calculations of molecular photoionization cross sections: N2 and PH3

A method based on the time-dependent density functional theory (TD-DFT) is proposed to calculate the photoionization cross section employing the explicit continuum wave-function, within a One Center Expansion and B-Splines radial basis set. The LB94 exchange-correlation potential with correct asymptotic behavior is employed. The results obtained for N2 and PH3 are in excellent agreement with the experimental data and are of comparable accuracy of ab initio methods. A deterioration is still present in the inner valence. For PH3 the effect of TD-DFT is dramatic and recovers completely the Kohn–Sham deficiency. The method has proven efficient for both valence and core ionization.

Density functional-time-dependent local density approximation calculations of autoionization resonances in noble gases

Absolute photoionization cross section profiles and asymmetry parameters of Ne, Ar and Kr have been calculated at the time-dependent local density approximation level. We employed a very accurate B-spline finite basis set and the modified Sternheimer approach, which is a first-order perturbative scheme particularly suited to finite basis set calculations. The gradient-dependent van Leeuwen and Baerends (VLB) exchange-correlation potential has been used, since it has the correct Coulombic behaviour at large distances which is a necessary condition for the existence of the Rydberg states. A simple modification of the boundary conditions considerably improves the number of terms of the Rydberg series. The resonance parameters obtained by fitting the calculated Fano profiles with a suitable analytic expression are compared with the experimental parameters.

Valence photoionization of C6H6 by the B-spline one-centre expansion density functional method

Abstract A density functional method based on a large-scale one-centre expansion in B-spline is employed to calculate the valence photoionization cross-section and the asymmetry parameter profiles in C 6 H 6 . The convergence of the method with increasing values of angular momentum is analysed. The partial cross-section and asymmetry parameter profiles are discussed for each orbital ionization. Information regarding the spatial extension of the wavefunction of the initial orbital can be gained by the cross-section contributions. Comparison with respect to a previous experiment and calculations is reported as well.

Density functional calculations of excitation energies and oscillator strengths for C1s → π∗ and O1s → π∗ excitations and ionization potentials in carbonyl containing molecules

Theoretical calculations of C1s → π∗ and O1s → π∗ excitation energies and oscillator strengths together with C1s and O1s ionization potentials in various carbonyl containing molecules have been performed. We employed the DF-LCAO method, with a DZP-STO basis set. Excitation and ionization energies are calculated by the transition state approach, and are more properly discussed as shifts with respect to CO. Comparison of the present results with accurate ab initio and experimental data available for free CO demonstrates the validity of such an approach. The other systems considered range from formylic fluorinated compounds (H2CO, HFCO and F2CO) to transition metal carbonyls (Cr(CO)6, Mo(CO)6, Fe(CO)5, Mn(CO)5Br, Mn(CO)5H and Ni(CO)4) and model molecules (NiCo, PdCO and PtCO). Particular attention is devoted to quantitative considerations about backbonding and to the capability of the excitation intensity to map the π∗ ground state orbitals. Comparison of the present theoretical results with experimental data demonstrates the high reliability of the method employed, which is able to correctly predict small effects (small energy shifts and differential relaxation). Some discrepancies in oscillator strengths are reasonably ascribed to experimental normalization errors.