Manipulating the Motion of Complex Molecules: Deflection, Focusing, and Deceleration of Molecular Beams for Quantum‐State and Conformer Selection

Abstract

Method summary Acronyms• effective dipole moment (µ eff )• low-field seeking (lfs) state• high-field seeking (hfs) state • Alternating gradient (AG) focusing• photo-multiplier tube (PMT)• transition state (TS) Strengths of technique• allows to spatially separate molecules in different quantum states, including isomers of complex molecules• versatile -DC field techniques can be applied to all polar molecules, AC field techniques to all polarizable molecules (= all molecules)• quantum-state specific interaction -can, in principle, be used to separate all kinds of isomers• determination of rotational, vibrational, and electronic properties -including dipole moments and polarizabilities -of single isomers of complex molecules• allows for detailed investigations of stereochemical dynamics• allows to separate molecular ensembles from seedgas, avoiding background in various scattering experiments Limitations• translations of non-polar molecules can -for practical purposes -not be manipulated by dc electric fields• no ultracold samples (µK or below) produced so far * This chapter is largely based on parts of reference 1. † Email: jochen.kuepper@cfel.de 1 2 Controlled moleculesChemists have long been dreaming of ultimately controlling all aspects of chemical reactions. Empirically, vast progress has been made over the last centuries using increasingly sophisticated techniques to dictate the path and outcome of chemical reactions. However, in all these approaches external parameters are used to (classically) shift statistical outcomes one way or another. Recently, the field of cold and ultracold chemistry has started to provide a glimpse at a new level of control, where full quantum-mechanical control can be obtained. So far this has been demonstrated for very specific small reactions systems, i. e., for reactions of alkali dimers with alkali atoms 2,3 or with each other, including the observation of stereochemical effects. 4 In these experiments molecules are prepared at low temperatures and specific quantum states starting from ultracold ensembles oftypically alkali -atoms, which is the limiting factor to the possible complexity and versatility of these methods. Alternatively, cold collisions of small molecules, i. e., OH radicals, with rare gas atoms at arbitrarily low collision energies have been investigated. 5,6 In this chapter we will present the available methods to gain control over the motion and the quantum-state populations over complex molecules (albeit at a lower level). These methods could enable a new level of detail in the study and control of chemical reactions for a large variety of molecules. Moreover, the prepared samples of controlled molecules are useful in a wide variety of experiments, ranging from the taking of actual photographs of the molecules -and their inherent or induced dynamics -using ultrafast X-ray or electron diffraction over, for example, photoelectron distributions and high-harmonic generation to investigations of attosecond electron dynamics and charge migration. 7 These experiments...

Three dimensional orientation of small polyatomic molecules excited by two-color femtosecond pulses

“…In addition, in the case of the formaldehyde molecule, the orientation lasts long after the end of the pulse. This long-lasting orientation of the dipole moment may, potentially, be probed by even-harmonic generation and may be useful for enhancing the deflection of molecular beams in the presence of inhomogeneous electrostatic fields [84]. Uniquely to chiral molecules, the induced orientation appears along the laser propagation direction when the polarizations of the FW and the SH are neither parallel nor orthogonal.…”

Long-Lasting Orientation of Symmetric-top Molecules Excited by Two-Color Femtosecond Pulses

“…Moreover, it has been demonstrated before that the degree of molecular alignment/orientation can be enhanced when a sequence of several laser pulses is used instead of a single pulse [17,[69][70][71][72][73][74], and a similar approach may be beneficial for increasing the degree of the long-lasting orientation. The orientation may be measured with the help of second (or higher-order) harmonic generation [42] and may be utilized in deflection experiments using inhomogeneous electrostatic fields [75,76]. We use free symmetric-top wave functions |JKM , the eigenfunctions of kinetic energy Hamiltonian, H r [55], as the basis set for numerical calculations.…”

Three Dimensional Orientation of Complex Molecules Excited by Two-Color Femtosecond Pulses

“…In addition, in the case of the formaldehyde molecule, the orientation lasts long after the end of the pulse. This long-lasting orientation of the dipole moment may, potentially, be probed by even-harmonic generation and may be useful for enhancing the deflection of molecular beams in the presence of inhomogeneous electrostatic fields [80]. Uniquely to chiral molecules, the induced orientation appears along the laser propagation direction when the polarizations of the fundamental wave and the second harmonic are neither parallel nor orthogonal.…”