Ultrafast X-ray Auger probing of photoexcited molecular dynamics.

Molecules can efficiently and selectively convert light energy into other degrees of freedom. Disentangling the underlying ultrafast motion of electrons and nuclei of the photoexcited molecule presents a challenge to current spectroscopic approaches. Here we explore the photoexcited dynamics of molecules by an interaction with an ultrafast X-ray pulse creating a highly localized core hole that decays via Auger emission. We discover that the Auger spectrum as a function of photoexcitation--X-ray-probe delay contains valuable information about the nuclear and electronic degrees of freedom from an element-specific point of view. For the nucleobase thymine, the oxygen Auger spectrum shifts towards high kinetic energies, resulting from a particular C-O bond stretch in the ππ* photoexcited state. A subsequent shift of the Auger spectrum towards lower kinetic energies displays the electronic relaxation of the initial photoexcited state within 200 fs. Ab-initio simulations reinforce our interpretation and indicate an electronic decay to the nπ* state.

Total photoionization cross-sections of excited electronic states by the algebraic diagrammatic construction-Stieltjes-Lanczos method.

Here, we extend the L2 ab initio method for molecular photoionization cross-sections introduced in Gokhberg et al. [J. Chem. Phys. 130, 064104 (2009)] and benchmarked in Ruberti et al. [J. Chem. Phys. 139, 144107 (2013)] to the calculation of total photoionization cross-sections of molecules in electronically excited states. The method is based on the ab initio description of molecular electronic states within the many-electron Green's function approach, known as algebraic diagrammatic construction (ADC), and on the application of Stieltjes-Chebyshev moment theory to Lanczos pseudospectra of the ADC electronic Hamiltonian. The intermediate state representation of the dipole operator in the ADC basis is used to compute the transition moments between the excited states of the molecule. We compare the results obtained using different levels of the many-body theory, i.e., ADC(1), ADC(2), and ADC(2)x for the first two excited states of CO, N2, and H2O both at the ground state and the excited state equilibrium or saddle point geometries. We find that the single excitation ADC(1) method is not adequate even at the qualitative level and that the inclusion of double electronic excitations for description of excited state photoionization is essential. Moreover, we show that the use of the extended ADC(2)x method leads to a substantial systematic difference from the strictly second-order ADC(2). Our calculations demonstrate that a theoretical modelling of photoionization of excited states requires an intrinsically double excitation theory with respect to the ground state and cannot be achieved by the standard single excitation methods with the ground state as a reference.

Total molecular photoionization cross-sections by algebraic diagrammatic construction-Stieltjes-Lanczos method: benchmark calculations.

In [K. Gokhberg, V. Vysotskiy, L. S. Cederbaum, L. Storchi, F. Tarantelli, and V. Averbukh, J. Chem. Phys. 130, 064104 (2009)] we introduced a new L(2) ab initio method for the calculation of total molecular photoionization cross-sections. The method is based on the ab initio description of discretized photoionized molecular states within the many-electron Green's function approach, known as algebraic diagrammatic construction (ADC), and on the application of Stieltjes-Chebyshev moment theory to Lanczos pseudospectra of the ADC electronic Hamiltonian. Here we establish the accuracy of the new technique by comparing the ADC-Lanczos-Stieltjes cross-sections in the valence ionization region to the experimental ones for a series of eight molecules of first row elements: HF, NH3, H2O, CO2, H2CO, CH4, C2H2, and C2H4. We find that the use of the second-order ADC technique [ADC(2)] that includes double electronic excitations leads to a substantial systematic improvement over the first-order method [ADC(1)] and to a good agreement with experiment for photon energies below 80 eV. The use of extended second-order ADC theory [ADC(2)x] leads to a smaller further improvement. Above 80 eV photon energy all three methods lead to significant deviations from the experimental values which we attribute to the use of Gaussian single-electron bases. Our calculations show that the ADC(2)-Lanczos-Stieltjes technique is a reliable and efficient ab initio tool for theoretical prediction of total molecular photo-ionization cross-sections in the valence region.

Multiphoton ionization as a clock to reveal molecular dynamics with intense short x-ray free electron laser pulses.

We investigate molecular dynamics of multiple ionization in N2 through multiple core-level photoabsorption and subsequent Auger decay processes induced by intense, short x-ray free electron laser pulses. The timing dynamics of the photoabsorption and dissociation processes is mapped onto the kinetic energy of the fragments. Measurements of the latter allow us to map out the average internuclear separation for every molecular photoionization sequence step and obtain the average time interval between the photoabsorption events. Using multiphoton ionization as a tool of the multiple-pulse pump-probe scheme, we demonstrate the modification of the ionization dynamics as we vary the x-ray laser pulse duration.

Ab initio interatomic decay widths of excited states by applying Stieltjes imaging to Lanczos pseudospectra.

Electronically excited states of atoms and molecules in an environment may decay in interatomic processes by transferring excess energy to neighboring species and ionizing them. The corresponding interatomic decay width is the most important characteristic of the decay allowing to calculate its efficiency and the final states' distribution. In this paper we present calculations of interatomic widths by the Fano-Stieltjes method applied to Lanczos pseudospectra, which has been previously shown to provide accurate autoionization widths in atoms and molecules. The use of Lanczos pseudospectra allows one to avoid the full diagonalization bottleneck and makes the method applicable to larger systems. We apply the present method to the calculation of interatomic decay widths in NeMg, NeAr and HCN[middle dot]Mg(n), n = 1, 2 clusters. The results are compared with widths obtained analytically and by other ab initio methods where available.

Radiationless decay in the region of the 2t2g and 4eg resonances in SF6.

The S 2p Auger spectrum of SF(6) has been studied in the region of the 2t(2g) and 4e(g) resonances. The partial Auger spectra due to the ionization of the 2p spin-orbit components and of a shake-up satellite state have been measured selectively by tuning the photon energy and using the Auger electron-photoelectron coincidence technique. A detailed analysis of the Auger spectrum has also been performed using the Green's function-based second-order algebraic diagrammatic construction method.

Autoionization widths by Stieltjes imaging applied to Lanczos pseudospectra.

Excited states of atoms and molecules lying above the ionization threshold can decay by electron emission in a process commonly known as autoionization. The autoionization widths can be calculated conveniently using Fano formalism and discretized atomic and molecular spectra by a standard procedure referred to as Stieltjes imaging. The Stieltjes imaging procedure requires the use of the full discretized spectrum of the final states of the autoionization, making its use for poly-atomic systems described by high-quality basis sets impractical. Following our previous work on photoionization cross-sections, here we show that also in the case of autoionization widths, the full diagonalization bottleneck can be overcome by the use of Lanczos pseudospectra. We test the proposed method by calculating the well-documented autoionization widths of inner-valence-excited neon and apply the new technique to autoionizing states of hydrofluoric acid and benzene.

Auger electron angular distribution of double core-hole states in the molecular reference frame.

The Linac Coherent Light Source free electron laser is a source of high brightness x rays, 2×10(11) photons in a ∼5 fs pulse, that can be focused to produce double core vacancies through rapid sequential ionization. This enables double core vacancy Auger electron spectroscopy, an entirely new way to study femtosecond chemical dynamics with Auger electrons that probe the local valence structure of molecules near a specific atomic core. Using 1.1 keV photons for sequential x-ray ionization of impulsively aligned molecular nitrogen, we observed a rich single-site double core vacancy Auger electron spectrum near 413 eV, in good agreement with ab initio calculations, and we measured the corresponding Auger electron angle dependence in the molecular frame.

Double core-hole production in N2: beating the Auger clock.

We investigate the creation of double K-shell holes in N2 molecules via sequential absorption of two photons on a time scale shorter than the core-hole lifetime by using intense x-ray pulses from the Linac Coherent Light Source free electron laser. The production and decay of these states is characterized by photoelectron spectroscopy and Auger electron spectroscopy. In molecules, two types of double core holes are expected, the first with two core holes on the same N atom, and the second with one core hole on each N atom. We report the first direct observations of the former type of core hole in a molecule, in good agreement with theory, and provide an experimental upper bound for the relative contribution of the latter type.

A photoelectron and double photoionization study of the valence electronic structure of 1,4-bromofluorobenzene.

Conventional photoelectron and time-of-flight photoelectron-photoelectron coincidence (TOF-PEPECO) spectra have been measured for the outer valence region of the 1,4-bromofluorobenzene molecule. The photoelectron spectra were recorded using HeI alpha radiation from a resonance source, and the TOF-PEPECO spectra were recorded using HeII alpha radiation from a pulsed resonance source. The former provide energies of the cationic states and the latter of the dicationic states. The spectra are adequately interpreted with the aid of accurate Green's function calculations, showing very significant correlation effects. The lowest double ionization energy is found at 23.45 eV associated with the (4b(1))(-2)X (1)A(1) dicationic state.

Molecular-beam scattering experiments and theoretical calculations probing charge transfer in weakly bound complexes of water.

We describe and analyze in depth a series of molecular beam scattering experiments, first reported by Aquilanti et al. (Angew. Chemie Int. Ed. 2005, 44, 2356.), proving that a measurable bond stabilization component beyond the van-der-Waals forces is present in the prototypal hydrophobic interaction of water with the noble gases (Ng). The experimental integral cross-section data, exhibiting a fully resolved "glory" interference pattern in the velocity dependence, are here quantitatively analyzed and characterized employing a recently proposed model potential. The stabilization component of the water-Ng bond has recently been attributed, through very accurate theoretical calculations and an unambiguous, model-free analysis of the electron density displacement, to a net electron transfer taking place from Ng to H(2)O. We review the theoretical analysis and discuss additional computational results, comparing them to experiment, that clarify the effect of charge transfer on the interaction energies.

Molecular photoionization cross sections by Stieltjes-Chebyshev moment theory applied to Lanczos pseudospectra.

Stieltjes imaging technique is widely used for the ab initio computation of photoionization cross sections and decay widths. The main problem hampering the application of the standard Stieltjes imaging algorithms in conjunction with high-level ab initio methods to polyatomic molecules is the requirement of full diagonalization of excessively large Hamiltonian matrices. Here we show that the full diagonalization bottleneck can be overcome by applying the Stieltjes imaging procedure to Lanczos pseudospectrum of the atomic or molecular Hamiltonian. Using the helium and neon atoms as examples, we demonstrate that the Lanczos pseudospectrum obtained after only a relatively small number of iterations can be used for Stieltjes-type calculations of photoionization cross sections essentially without loss of accuracy. The new technique is applied to the calculation of the total photoionization cross section of benzene within an ab initio approach explicitly taking into account single and double electronic excitations. Good agreement with experimental results is obtained.

The Auger spectroscopy of pyrimidine and halogen-substituted pyrimidines.

The C 1s and N 1s Auger spectra of pyrimidine, 2-chloropyrimidine, and 5-bromopyrimidine have been measured in an electron impact experiment at 1000 eV. In the case of the halogen-substituted pyrimidines, also the Cl 2p and Br 3d Auger spectra have been recorded. We have thoroughly analyzed and interpreted all the Auger spectra recorded here with the aid of accurate Green's function calculations with a large basis set. The spectra are extremely complex with thousands of states contributing and almost no single-state feature even near the double ionization threshold. Besides reproducing and explaining with great detail nearly all the main spectral features observed, the calculations have successfully unraveled the interplay among the different C 1s core hole chemical shifts in each molecule and how this affects some fingerprinting details in the composite C 1s Auger spectra.

Double photoionization of thiophene and bromine-substituted thiophenes.

We report the double photoionization spectra of thiophene, 3-bromothiophene, and 3,4-dibromothiophene using a coincidence spectroscopy technique based on electron time-of-flight measurements. Spectra have been recorded between the onset and 40.814 eV using He IIalpha radiation. The He I photoelectron spectrum of 3,4-dibromothiophene has also been measured. All the spectra have been analyzed and interpreted in detail on the basis of theoretical simulations from accurate Green's function calculations.

An experimental and theoretical study of double photoionization of CF4 using time-of-flight photoelectron-photoelectron (photoion-photoion) coincidence spectroscopy.

Single photon double ionization of CF4 has been studied by means of a time-of-flight photoelectron-photoelectron coincidence technique, which has very recently been extended towards ion detection, with energy analysis for the electrons and mass analysis for the ions. The complete single photon double ionization electron spectrum of CF4 up to a binding energy of approximately 51 eV is presented and discussed, also with the aid of accurate ab initio Green's function calculations. From ion detection in coincidence with the ejected electrons, we derive fragmentation pathway-selected double ionization electron spectra of CF4. From the same data we extract the yield of each doubly charged ion or ion pair as a function of the double ionization energy.

Effects of nuclear dynamics in the low-kinetic-energy Auger spectra of CO and CO2.

The CO and CO(2) carbon and oxygen Auger spectra have been measured by electron impact and compared with accurate theoretical calculations accounting for the effects of the dynamics of the nuclei on the energy and linewidth of the Auger bands. The calculations for CO were previously published [L. S. Cederbaum et al., J. Chem. Phys. 95, 6634 (1991)], while for CO(2) they are new and presented here for the first time. For both molecules, particular attention has been paid to the low-kinetic-energy region of the spectra, which corresponds to doubly charged ion states with the two holes mainly localized in the inner valence region. New bands have been observed. It is shown that a proper consideration of the vibrational broadening and shift of the bands due to the dynamics of the nuclei is needed to assign these features. For CO, very large energy shifts between corresponding features in the C 1s and O 1s spectra have been observed, confirming the theoretical predictions of 1991. The new computed spectra of CO(2) allow a very accurate analysis of the experiments over the whole energy range.

Complete valence double photoionization of SF6.

Single photon double ionization of SF(6) has been investigated at the photon energies 38.71, 40.814, and 48.372 eV by using a recently developed time-of-flight photoelectron-photoelectron coincidence spectroscopy technique which gives complete two-dimensional e(-)-e(-) spectra. The first complete single photon double ionization electron spectrum of SF(6) up to a binding energy of approximately 48 eV is presented and accurately interpreted with the aid of Green's function ADC(2) calculations. Spectra which reflect either mainly direct or mainly indirect (via interatomic coulombic decay of F 2s holes) double ionization of SF(6) are extracted from the coincidence map and discussed. A previous, very low value for the onset of double ionization of SF(6) is found to energetically coincide with a peak structure related to secondary inelastic scattering events.