Polyatomic Molecules under Intense Femtosecond Laser Irradiation

Abstract

Interaction of intense laser pulses with atoms and molecules is at the forefront of atomic, molecular, and optical physics. It is the gateway to powerful new tools that include above threshold ionization, high harmonic generation, electron diffraction, molecular tomography, and attosecond pulse generation. Intense laser pulses are ideal for probing and manipulating chemical bonding. Though the behavior of atoms in strong fields has been well studied, molecules under intense fields are not as well understood and current models have failed in certain important aspects. Molecules, as opposed to atoms, present confounding possibilities of nuclear and electronic motion upon excitation. The dynamics and fragmentation patterns in response to the laser field are structure sensitive; therefore, a molecule cannot simply be treated as a "bag of atoms" during field induced ionization. In this article we present a set of experiments and theoretical calculations exploring the behavior of a large collection of aryl alkyl ketones when irradiated with intense femtosecond pulses. Specifically, we consider to what extent molecules retain their molecular identity and properties under strong laser fields. Using time-of-flight mass spectrometry in conjunction with pump-probe techniques we study the dynamical behavior of these molecules, monitoring ion yield modulation caused by intramolecular motions post ionization. The set of molecules studied is further divided into smaller sets, sorted by type and position of functional groups. The pump-probe time-delay scans show that among positional isomers the variations in relative energies, which amount to only a few hundred millielectronvolts, influence the dynamical behavior of the molecules despite their having experienced such high fields (V/Å). High level ab initio quantum chemical calculations were performed to predict molecular dynamics along with single and multiphoton resonances in the neutral and ionic states. We propose the following model of strong-field ionization and subsequent fragmentation for polyatomic molecules: Single electron ionization occurs on a suboptical cycle time scale, and the electron carries away essentially all of the energy, leaving behind little internal energy in the cation. Subsequent fragmentation of the cation takes place as a result of further photon absorption modulated by one- and two-photon resonances, which provide sufficient energy to overcome the dissociation energy. The proposed hypothesis implies the loss of a photoelectron at a rate that is faster than intramolecular vibrational relaxation and is consistent with the observation of nonergodic photofragmentation of polyatomic molecules as well as experimental results from many other research groups on different molecules and with different pulse durations and wavelengths.

Mechanisms and time-resolved dynamics for trihydrogen cation (H3 +) formation from organic molecules in strong laser fields

“…To obtain time-resolved information, the laser pulse was split into pump and probe pulses in a Mach-Zehnder geometry. More information regarding the setup and data acquisition can be found elsewhere 29, 41 . Residual high-order dispersions in the amplified laser pulses were removed using the multiphoton intrapulse interference phase scan (MIIPS) technique 42, 43 .…”

“…To accurately determine the formation times for each pathway, we conducted pump-probe time-resolved measurements with femtosecond temporal resolution 29 . Please see the Methods: Experimental Setup section for additional details.…”

Coherent Vibrational and Dissociation Dynamics of Polyatomic Radical Cations

“…In contrast, the presence of multiple coupled normalm odes leads to fast dephasing of initially prepared vibrational wave packets in polyatomic radical cations within atm ost several picoseconds of the initial ionization event. Nevertheless, coherenti on yield oscillations have been observed in many classes of polyatomic radical cations including halomethanes, [32,33] 1,3-dibromopropane, [34] azobenzene, [35] alkyl aryl ketones, [21,36,37] nitrotoluenes, [27,38] and methylphosphonates. [22,39] Although in most cases, the ion yield oscillations are attributed to dynamics on D 0 ,d ynamics on excited ionic states have also been reported.…”

Conserving Coherence and Storing Energy during Internal Conversion: Photoinduced Dynamics of cis- and trans-Azobenzene Radical Cations

“…[1][2][3][4][5][6][7] Recently, this technique has been increasingly applied to studying the post-ionization dynamics of radical cations. [8][9][10][11][12][13][14][15][16][17] A major challenge in these experiments stems from the fact that ionization via multiphoton absorption typically produces vibrationally hot ions. 10 This results in extensive fragmentation, which makes it difficult to actually probe the dynamics of the intact parent ion.…”

“…10,18 A solution to this problem was found in employing strong-field adiabatic ionization by long-wavelength (IR) laser pulses, which was shown to create 3 vibrationally cold cations and thus effectively open up the possibility for controlling the fragmentation channels. 12,17,18 The interaction of cis-and trans-azobenzene with laser radiation in both the gas and liquid phases have been studied extensively with femtosecond spectroscopy, [19][20][21][22] and it is well established that neutral azobenzene undergoes photoisomerization when excited to S 1 (n,π*) or to one or more higher lying π* excited states S 2/3/4 (π,π*). 22 Studies of the ionized species are however sparse.…”