Femtosecond Real-Time Probing of Reactions. 18. Experimental and Theoretical Mapping of Trajectories and Potentials in the NaI Dissociation Reaction

Perspective: Preservation of coherence in photophysical processes

Coherence is one of the most important phenomena in ultrafast sciences. We give our perspective on the terminology, observation, and preservation of coherence in photophysical processes with some glimpses to the past and some looking-head to what may pave the way for scaling one of the last bastions in ultrafast science, namely, that of mode specific chemistry where it will be possible to break any specific bond by tailoring the pulse, an accomplishment that obviously would be the dream of any chemist.

Vibrational coherence transfer in the ultrafast intersystem crossing of a diplatinum complex in solution

We investigate the ultrafast transient absorption response of tetrakis(μ-pyrophosphito)diplatinate(II), [Pt₂(μ-P₂O₅H₂)₄]^4- [hereafter abbreviated Pt(pop)], in acetonitrile upon excitation of its lowest singlet ¹A_2u state. Compared with previously reported solvents [van der Veen RM, Cannizzo A, van Mourik F, Vlček A, Jr, Chergui M (2011) J Am Chem Soc 133:305-315], a significant shortening of the intersystem crossing (ISC) time (<1 ps) from the lowest singlet to the lowest triplet state is found, allowing for a transfer of vibrational coherence, observed in the course of an ISC in a polyatomic molecule in solution. Density functional theory (DFT) quantum mechanical/molecular mechanical (QM/MM) simulations of Pt(pop) in acetonitrile and ethanol show that high-lying, mostly triplet, states are strongly mixed and shifted to lower energies due to interactions with the solvent, providing an intermediate state (or manifold of states) for the ISC. This suggests that the larger the solvation energies of the intermediate state(s), the shorter the ISC time. Because the latter is smaller than the pure dephasing time of the vibrational wave packet, coherence is conserved during the spin transition. These results underscore the crucial role of the solvent in directing pathways of intramolecular energy flow.

Predissociation dynamics of lithium iodide

The predissociation dynamics of lithium iodide (LiI) in the first excited A-state is investigated for molecules in the gas phase and embedded in helium nanodroplets, using femtosecond pump-probe photoionization spectroscopy. In the gas phase, the transient Li(+) and LiI(+) ion signals feature damped oscillations due to the excitation and decay of a vibrational wave packet. Based on high-level ab initio calculations of the electronic structure of LiI and simulations of the wave packet dynamics, the exponential signal decay is found to result from predissociation predominantly at the lowest avoided X-A potential curve crossing, for which we infer a coupling constant VXA = 650(20) cm(-1). The lack of a pump-probe delay dependence for the case of LiI embedded in helium nanodroplets indicates fast droplet-induced relaxation of the vibrational excitation.

Competitive direct vs. indirect photochromism dynamics of constrained inverse dithienylethene molecules

Relaxation observed through several parallel pathways from the first excited state to the ground-state in inverse dithienylethene molecules.

NONADIABATIC EFFECTS IN FEMTOSECOND PHOTOIONIZATION OF NaI MOLECULE

In this work, the nonadiabatic effects in the femtosecond photoionization of NaI molecule driven by pump and probe pulses are investigated theoretically using the accurate quantum wave packet method. The calculation with the inclusion of nonadiabatic coupling remarkably improves the agreement with experimental results of Jouvet et al. [J Phys Chen A101: 2555, 1997], indicating the importance of the nonadiabatic effect. Moreover, the dynamical evolutions of wave packets and their corresponding time-resolved photoelectron spectroscopy are presented both on the adiabatic and diabatic potentials. These comparisons contribute to further understanding of the influences of nonadiabatic effects in the femtosecond photoionization of NaI molecule.

Coherent oscillatory femtosecond dynamics in multichannel photodynamics of NO2 studied by spatially masked electron imaging

The femtosecond multiphoton photoionization and dissociation dynamics of NO(2) have been studied in a two-color pump-probe experiment at 400 and 266 nm using velocity map ion imaging in conjunction with photoelectron imaging. We report here a series of experiments focusing on the oscillatory patterns in pump-probe transients of the photoelectron signal. By using the technique of spatially masked imaging detection, we can select different photoelectron channels enabling the rapid measurement of energy selected transients with good signal-to-noise ratio. At short delay times (<300 fs) the dominant process is dissociative multiphoton ionization by 3 x 400 nm + 1 x 266 nm excitation to a repulsive potential energy surface of the NO(2)(+) cation correlating to NO(+)((1)Sigma(+)) + O((3) P) and the ejection of a 0.37 eV electron. At longer delay times (>400 fs), the release of high-energy electrons (0.88 eV) is observed attributed to a three-photon absorption at 400 nm to Rydberg and valence type excited states of neutral NO(2) leading to predissociation and the production of NO(+) + O((3)P) from a one-photon ionization at 266 nm. At longer delay times (>400 fs) a second slow (near 0 eV) photoelectron channel is observed that is associated with one photon excitation at 400 nm to the first excited A(2)B(2) state of NO(2) followed by two-photon excitation at 266 nm leading to near threshold ionization and dissociation to NO(+) + O((3)P). Distinctive oscillatory patterns were found in the pump-probe transients of the photoelectron yield for both the slow and the fast photoelectron channels but with different periods of about 750 fs (slow) or 590 fs (fast). Extensive polarization experiments are reported for both linear and circular polarized pump and probe laser geometries. We discuss the oscillatory mechanism in relation to ab initio calculations of relevant Rydberg and valence type excited states of NO(2) near 9.3 eV. We propose that an oscillating wavepacket of mixed Rydberg and valence character that predissociates is responsible for the observed oscillations in the transients of the fast (0.88 eV) photoelectron channel.

Ultrafast nonadiabatic dynamics of [Fe(II)(bpy)(3)](2+) in solution

The ultrafast relaxation of aqueous iron(II)-tris(bipyridine) upon excitation into the singlet metal-to-ligand charge-transfer band (1MLCT) has been characterized by femtosecond fluorescence up-conversion and transient absorption (TA) studies. The fluorescence experiment shows a very short-lived broad 1MLCT emission band at approximately 600 nm, which decays in < or =20 fs, and a weak emission at approximately 660 nm, which we attribute to the 3MLCT, populated by intersystem crossing (ISC) from the 1MLCT state. The TA studies show a short-lived (<150 fs) excited-state absorption (ESA) below 400 nm, and a longer-lived one above 550 nm, along with the ground-state bleach (GSB). We identify the short-lived ESA as being due to the 3MLCT state. The long-lived ESA decay and the GSB recovery occur on the time scale of the lowest excited high-spin quintet state 5T2 lifetime. A singular value decomposition and a global analysis of the TA data, based on a sequential relaxation model, reveal three characteristic time scales: 120 fs, 960 fs, and 665 ps. The first is the decay of the 3MLCT, the second is identified as the population time of the 5T2 state, while the third is its decay time to the ground state. The anomalously high ISC rate is identical in [RuII(bpy)3]2+ and is therefore independent of the spin-orbit constant of the metal atom. To reconcile these rates with the regular quasi-harmonic vibrational progression of the 1MLCT absorption, we propose a simple model of avoided crossings between singlet and triplet potential curves, induced by the strong spin-orbit interaction. The subsequent relaxation steps down to the 5T2 state dissipate approximately 2000 cm-1/100 fs. This rate is discussed, and we conclude that it nevertheless can be described by the Fermi golden rule, despite its high value.

Interpreting closed-loop learning control of molecular fragmentation in terms of wave-packet dynamics and enhanced molecular ionization

We interpret a molecular fragmentation experiment using shaped, ultrafast laser pulses in terms of enhanced molecular ionization during dissociation. A closed-loop learning control experiment was performed to maximize the CF3+CH3+ production ratio in the dissociative ionization of CH3COCF3. Using ab inito molecular structure calculations and quasistatic molecular ionization calculations along with data from pump-probe experiments, we identify the primary control mechanism which is quite general and should be applicable to a broad class of molecules.

Pump-probe photoionization study of the passage and bifurcation of a quantum wave packet across an avoided crossing

The application of femtosecond pump-probe photoelectron spectroscopy to directly observe vibrational wave packets passing through an avoided crossing is investigated using quantum wave packet dynamics calculations. Transfer of the vibrational wave packet between diabatic electronic surfaces, bifurcation of the wave packet, and wave packet construction via nonadiabatic mixing are shown to be observable as time-dependent splittings of peaks in the photoelectron spectra.

Surface femtochemistry: observation and quantum control of frustrated desorption of alkali atoms from noble metals

This review presents a case study of the direct, real-time observation of a surface photochemical reaction, namely the frustrated photodesorption of alkali atoms from noble metal surfaces. Charge transfer excitation of an electron from the metal substrate into an unoccupied resonance of the alkali atom instantaneously turns on the repulsive Coulomb force inducing the nuclear motion of both the adsorbate and substrate atoms. The incipient nuclear wave packet dynamics are documented for the case of Cs/Cu(111) through the accompanying change in the surface electronic structure. The intimate view of atoms attempting to escape the surface bond highlights the unique role of the substrate in the electronic and nuclear dynamics that ultimately determine the product yields. Moreover, slow dephasing of the coherent polarization is exploited to demonstrate the control of nuclear wave packets through the phase of the excitation light.