The development of femtosecond time-resolved methods for the study of gas-phase molecular dynamics is founded upon the seminal studies of Zewail and co-workers, as recognized in 1999 by the Nobel Prize in Chemistry.1 This methodology has been applied to chemical reactions ranging in complexity from bond-breaking in diatomic molecules to dynamics in larger organic and biological molecules, and has led to breakthroughs in our understanding of fundamental chemical processes. Photoexcited polyatomic molecules and anions often exhibit quite complex dynamics involving the redistribution of both charge and energy.2-6 These processes are the primary steps in the photochemistry of many polyatomic systems,7 are important in photobiological processes such as vision and photosynthesis,8 and underlie many concepts in molecular electronics.9 Femtosecond time-resolved methods involve a pump-probe configuration in which an ultrafast pump pulse initiates a reaction or, more generally, creates a nonstationary state or wave packet, the evolution of which is monitored as a function of time by means of a suitable probe pulse. Time-resolved or wave packet methods offer a view complementary to the usual spectroscopic approach and often yield a physically intuitive picture. Wave packets can behave as zeroth-order or even classical-like states and are therefore very helpful in discerning underlying dynamics. The information obtained from these experiments is very much dependent on the nature of the final state chosen in a given probe scheme. Transient absorption and nonlinear wave mixing are often the methods of choice in condensed-phase experiments because of their generality. In studies of molecules and clusters in the gas phase, the most popular methods, laser-induced fluorescence and resonant multiphoton ionization, usually require the probe laser to be resonant with an electronic transition in the species being monitored. However, as a chemical reaction initiated by the pump pulse evolves toward products, one expects that both the electronic and vibrational structures of the species under observation will change. Hence, these probe methods can be * To whom corresondence should be addressed. A.S.: telephone (613) 993-7388, fax (613) 991-3437, E-mail albert.stolow@nrc.ca. D.M.N.: telephone (510) 642-3505, fax (510) 642-3635, E-mail dan@radon.cchem.berkeley.edu. 1719 Chem. Rev. 2004, 104, 1719−1757

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Femtosecond time-resolved photoelectron spectroscopy.

Time-resolved photoelectron spectroscopy (TRPES) has become a powerful new tool in studying the dynamics of molecules and clusters. It has been applied to processes ranging from energy flow in electronically excited states of molecules to electron solvation dynamics in clusters. This review covers experimental and theoretical aspects of TRPES, focusing on studies of neutral and negatively charged species.

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Time-resolved photoelectron spectroscopy of molecules and clusters

Comprehensive studies of the molecular and electronic structures, vibrational frequencies, and infrared and Raman intensities of the aniline radical cation, C6H5NH2+ have been performed by using the unrestricted density functional (UB3LYP) and second-order Moller–Plesset (UMP2) methods with the extended 6-311++G(df,pd) basis set. For comparison, analogous calculations were carried out for the closed-shell neutral aniline. The studies provided detailed insight into the bonding changes that take place in aniline upon ionization. The natural bond orbital (NBO) analysis has revealed that the pπ-radical conjugative interactions are of prime importance in stabilizing the planar, quinoid-type structure of the aniline radical cation. It is shown that the natural charges calculated for aniline are consistent with the chemical properties of this molecule (an ortho- and para-directing power of the NH2 group in electrophilic substitutions), whereas Mulliken charges are not reliable. The theoretical vibrational frequencies of aniline, calculated by the B3LYP method, show excellent agreement with the available experimental data. In contrast, the MP2 method is deficient in predicting the frequencies of several modes in aniline, despite the use of the extended basis set in calculations. The frequencies of aniline radical cation, calculated at the UB3LYP/6-311++G(df,pd) level, are in very good agreement with the recently reported experimental data from zero kinetic energy photoelectron and infrared depletion spectroscopic studies. The clear- cut assignment of the IR and Raman spectra of the investigated molecules has been made on the basis of the calculated potential energy distributions. Several bands in the spectra have been reassigned. It is shown that ionization of aniline can be easily identified by the appearance of the very strong band at about 1490 cm−1, in the Raman spectrum. The redshift of the N–H stretching frequencies and the blueshift of the C–H stretching frequencies are observed in aniline, upon ionization. As revealed by NBO analysis, the frequency shifts can be correlated with the increase of electron density (ED) on the antibonding orbitals (σNH*) and decrease of ED on σCH*, respectively. These effects are associated with a weakening of N–H bonds and strengthening of C–H bonds in the aniline radical cation. The simulated theoretical Raman and infrared spectra of aniline and its radical cation, reported in this work, can be used in further spectroscopic studies of their van der Waals clusters and hydrogen bonded complexes.

Electronic structures, vibrational spectra, and revised assignment of aniline and its radical cation: Theoretical study

▪ Abstract The use of photoelectron angular distributions (PADs) as a probe in short-pulse, pump-probe scenarios is reviewed. We focus on concepts, on the insight that can be gained through theoretical analysis, on applications, and on future opportunities. Time-resolved PADs are sensitive to both the time-evolving rotational composition of wavepackets and their time-evolving electronic symmetry. The former feature renders this observable a potential probe of molecular structure, intensity effects, and rotational perturbations. The latter feature renders the PAD a potential probe of radiationless transitions.

TIME-RESOLVED PHOTOELECTRON ANGULAR DISTRIBUTIONS: Concepts, Applications, and Directions

In this work, we present an exhaustive study of the structural as well as the electronic properties of aniline obtained using several DFT based methods. The geometry of aniline obtained using the hybrid DFT methods (B3LYP, B3P86 and B3PW91) shows better agreement with the experimental values, than the methods containing the gradient corrected exchange functional (Becke 88). The new feature of the hybrid methods is that the exact Hartree–Fock exchange functional was taken in account. Negative highest occupied and lowest unoccupied Kohn–Sham orbitals (we shall call them as HOMO, LUMO) seem to provide a fair estimation of the experimental IPs and EAs values (particularly those obtained by using the gradient correct exchange functional Becke 88). IPs and EAs from DFT calculations present systematic errors are observed owing to the HOMO and LUMO, which do not correspond formally to any molecular orbital.We, also, study in a systematic way the influence of a wide range of σ–π electron donor-withdrawing substituent groups on the structural and electronic properties of aniline. In this kind of systems, the position of the substituent group in the benzene ring as well as its electron donor–acceptor capabilities play a very important role on the molecular structural and electronic properties. We also find that the intramolecular hydrogen bonds are responsible for many particular experimental features.

Density functional studies of aniline and substituted anilines

Zero kinetic energy electron spectra of aniline are recorded and a number of ion vibrational frequencies accurately measured. The adiabatic ionization potential of aniline is found to be 62 271±2 cm−1. Ab initio calculations provide insights into the bonding changes that take place in aniline upon ionization. Reasonable agreement between calculated and measured frequencies in the ground states of aniline neutral and ion is obtained.

Zero kinetic energy photoelectron spectra of jet‐cooled aniline

Direct observation of time-dependent intramolecular vibrational relaxation in jet-cooled p-n-alkylanilines by laser photoelectron spectroscopy

In order to study the effect of photoelectron kinetic energy on rotational photoionization propensity rules, rotationally resolved laser photoelectron spectra were measured for excitation of specific rovibronic levels in the D 2Sigma+ (3psigma) Rydberg state of NO and their subsequent ionization by radiation at several wavelengths. The measured and calculated ion rotational branching ratios both show a significant dependence on photoelectron energy. Comparison between experimental data and theoretical calculations suggests that a strong DeltaN=0 peak in the spectra is caused by an interaction between particular vibronic levels of the A 2Sigma+ (v=4) and D 2Sigma+ (v=0) Rydberg states.

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Dependence of NO rotational photoionization propensity rules on electron kinetic energy

ZEKE spectra of propylaniline were recorded by exciting 13 vibronic levels in the S1 electronic state. All ZEKE spectra show well resolved vibrational structure of the anti and gauche cations. Adiabatic ionization potentials of the anti and gauche conformations were determined to be 59 717±3 cm−1 (7.4040 eV) and 59 793±3 cm−1 (7.4134 eV), respectively. A number of ring modes as well as several low‐frequency torsional and bending chain modes have been identified. Spectra demonstrate that low‐frequency chain modes are sensitive to the molecular conformation. Ab initio calculations were performed to understand the nature of these low‐frequency chain modes. S1←S0 vibronic transitions involving anti and gauche conformations can easily be distinguished using the ZEKE method.

Xinbei Song

Papers

Zero kinetic energy photoelectron spectra of jet‐cooled aniline

Direct observation of time-dependent intramolecular vibrational relaxation in jet-cooled p-n-alkylanilines by laser photoelectron spectroscopy

Dependence of NO rotational photoionization propensity rules on electron kinetic energy

High‐resolution zero kinetic energy photoelectron spectra of para‐n‐propylaniline

Dependence of p-n-propylaniline ionization potential on molecular conformation : comparison of experiment with theory