Field-Free Three-Dimensional Alignment of Polyatomic Molecules

Abstract

We experimentally demonstrate field-free, three-dimensional alignment (FF3DA) of polyatomic asymmetric top molecules. We achieve FF3DA in sulfur dioxide gas using two time-delayed, orthogonally polarized, nonresonant, femtosecond laser pulses. Our method avoids the use of rotational revivals and is therefore more robust to temperature. The alignment is probed using time-delayed coincidence Coulomb explosion imaging. FF3DA will be important for all molecular imaging, dynamics, or spectroscopy experiments for which random alignment leads to a loss of information. DOI: 10.1103/PhysRevLett.97.173001 PACS numbers: 33.15.Bh, 33.55.Be, 33.80.ÿb The random alignment of gas phase molecules generally reduces the information content of a measurement made in the laboratory frame. The situation is analogous to the well-known case of powder, as opposed to crystal, x-ray diffraction. To ameliorate this situation, it is necessary to define the direction of the molecules in the lab frame prior to making a measurement. Polyatomic molecules are generally asymmetric rotors with three distinct axes of rotation. Three-dimensional alignment, i.e., the alignment of all three molecular axes, was achieved in the presence of an aligning laser field [1,2], but this field strongly perturbs the system, distorting the electronic and vibrational structure of the molecule [3] and preventing the measurement of innate molecular properties. Field-free one-dimensional alignment, i.e., alignment of a single molecular axis, was achieved using a short laser pulse [4,5], and by the rapid turn off of an adiabatic strong laser field [6]. In these experiments, maximal alignment is produced after the pulse [4,7] when the molecule is field-free, followed by periodic revivals of the rotational wave packet [4]. The rotational energy level spacings for asymmetric tops are much less regular than for linear and symmetric top molecules. This complicates the rotational wave-packet evolution for asymmetric rotors, reducing alignment at revivals. The degradation of alignment worsens at elevated rotational temperatures due to the incoherent contributions of thermally populated rotational states which can lead to complete obfuscation of the rotational revival structure. This is a challenge for experimentalists since rotational cooling is often compromised in order to produce sufficiently dense molecular beams.Here we report the experimental demonstration of a general, flexible, and robust method for producing fieldfree, three-dimensional alignment (FF3DA) of polyatomic molecules. This method makes use of the prompt alignment occurring just after the laser pulse and is much more robust with respect to temperature effects than is an earlier proposal for FF3DA at rotational revivals [8]. This technique is broadly applicable and we demonstrate it here for the asymmetric top molecule SO 2 .Our method is based upon the use of two time-separated, perpendicularly polarized, nonresonant, femtosecond laser pulses. The first laser pulse produces postpulse 1D alignment of the...

Laser-induced 3D alignment and orientation of quantum state-selected molecules

“…The ideal situation is to obtain the 3D orientation under field-free conditions. This may be possible by, for instance, using timed perpendicularly polarized short pulses [13] or by turning off the long YAG pulse rapidly compared to the inherent molecular rotation times. In the case of 1D alignment and orientation it has been shown that such a conversion from adiabatic alignment/orientation to field-free alignment/orientation is possible by femtosecond truncation of the YAG pulse [33][34][35].…”

“…Rather, all three molecular axes of polarisability must be * Electronic address: henriks@chem.au.dk † Electronic address: jochen@fhi-berlin.mpg.de confined to laboratory fixed axes. Such 3-dimensional (3D) alignment has been experimentally demonstrated through the use of an elliptically polarized laser field [8][9][10][11] or by combining two linearly polarized laser pulses with orthogonal polarizations [12][13][14][15].…”

Influence of the repulsive Coulomb barrier on photoelectron spectra and angular distributions in a resonantly excited dianion

“…19,20 However, with no prior knowledge of the detailed differential photodetachment cross-section, the degree of observed anisotropy is convoluted with this unknown and so can be difficult to interpret. To overcome this, the laboratory and molecular frames can be connected through molecular alignment, 43,44 as discussed below. (Fig.…”

Linked attosecond phase interferometry for molecular frame measurements

“…Accessing these quantities is especially important for the development of single-molecule imaging by high harmonic generation [10][11][12][13]. LAPIN is applicable to any molecular systems that can be expanded in the gaseous phase, these include: aligned [20], three-dimensionally aligned [29] and oriented [30] polyatomic molecules. Finally, combining…”