Femtosecond Coincidence Imaging of Multichannel Multiphoton Dynamics

Abstract

The novel technique of femtosecond time-resolved photoelectron-photoion coincidence imaging is applied to unravel dissociative ionization processes in a polyatomic molecule. Femtosecond coincidence imaging of CF 3 I photodynamics illustrates how competing multiphoton dissociation pathways can be distinguished, which would be impossible using photoelectron or ion imaging alone. Ion-electron energy correlations and photoelectron angular distributions reveal competing processes for the channel producing (e ÿ CF 3 I). The molecular-frame photoelectron angular distributions of the two major pathways are strikingly different. DOI: 10.1103/PhysRevLett.92.123002 PACS numbers: 33.50.Hv, 33.60.Cv, 33.80.Eh, 33.80.Gj Multiphoton excitation in molecular photodynamics enables studies of highly excited states and their complex decay dynamics. Intense femtosecond laser pulses easily induce multiphoton excitation and provide the time resolution needed to study the dynamics. Multiphoton excitation resulting in ionization frequently reveals multiple, complex ionization and fragmentation channels that are not seen in single-photon ionization. Compared to singlephoton ionization, the multiphoton excitation accesses different Franck-Condon factors via resonant intermediate states and changes the probability of multielectron excitation processes. The energetics and vector correlations of dissociative ionization processes can be studied in complete detail using recently developed coincident imaging techniques [1]. Different energetically accessible dissociation channels can be identified from photoelectron-photoion energy correlations. Vector correlations, such as photoelectron angular distributions (PADs), provide information on the symmetry, electronic configuration, and orientation of the molecule as the electron is ejected [2 -5]. So far, photoelectron-photoion coincident (PEPICO) imaging techniques have been mostly applied in ionization studies using one-photon excitation with synchrotron or He(I) radiation [6 -11]. Only recently, first femtosecond time-resolved PEPICO imaging experiments were reported [12,13], and new theoretical frameworks have been developed to study time-resolved photoelectron dynamics [14,15]. In this Letter we use the full potential of time-resolved coincidence imaging to unravel competing ionization and fragmentation processes in multiphoton excited CF 3 I.The CF 3 I molecule is investigated because it exhibits a variety of interesting dissociative ionization processes. It is one of the few polyatomic molecules for which these processes have been previously explored by both single and multiphoton excitation [6,7,16 -18]. In particular, Powis and co-workers have used PEPICO imaging coupled with single-photon ionization sources [e.g. He(I)] to probe the dissociation of theà A 2 A 1 state of CF 3 I and measure molecular-frame PADs (MF-PADs) associated with the CF 3 and I fragments [6]. Here we apply femtosecond time-resolved multiphoton excitation of CF 3 I to the region just below theà A 2 A 1 state of C...

Ionization of one- and three-dimensionally-oriented asymmetric-top molecules by intense circularly polarized femtosecond laser pulses

“…This is possible if ionization leads to fragmentation of the molecule and the electrons can be measured in coincidence with fragment ions recoiling in directions that directly reflect the molecular orientation at the moment of ionization, a condition often referred to as the axial recoil limit. Such coincidence methods have been applied successfully, predominantly to small molecules where synchrotron radiation removes an inner shell electron [6][7][8][9] but also to situations where valence electrons are removed by UV radiation [10][11][12].…”

A detailed experimental and theoretical study of the femtosecond A-band photodissociation of CH3I

“…3,8,[19][20][21] Femtosecond imaging experiments include photoelectron 3,22 and photoionphotoelectron coincidence detection. 3,6,7,22 Here we have studied the femtosecond photodissociation dynamics of CH 3 I from the A band both experimentally and theoretically. Experimentally, the velocity map imaging technique, which provides selectivity at the detection end, has been combined with tunable UV femtosecond laser radiation, which provides selectivity and resonant enhancement at the probe end by resonance enhanced multiphoton ionization ͑REMPI͒ of the products.…”

“…The experimental observation of chemical transformations in real time is the focus of femtochemistry, which utilizes ultrashort laser pulses to diagnose the time evolution of a chemical reaction. Two decades after the birth of this new field in chemistry 1,2 and with a wealth of accumulated information concerning molecular dynamics [3][4][5][6][7][8][9] even in very complex, often biologically relevant systems, 3,[10][11][12][13][14] basic questions about the real time dynamics of simple systems still remain to be explored. It is only via the combination of recently developed detection techniques and the full exploitation of stateof-the-art femtosecond laser systems that we can hope to address them.…”

Femtosecond time-resolved photoelectron-photoion coincidence imaging of multiphoton multichannel photodynamics in NO2

“…[14][15][16][17][18][19] The femtosecond time-resolved photoelectron-photoion coincidence imaging technique was pioneered by Hayden and co-workers and applied to NO 2 , 20,21 CF 3 I, 22 and the NO dimer. 23 In particular, Hayden and co-workers studied the femtosecond dynamics of NO 2 in a single color pump-probe experiment at 375 nm.…”