Visualizing the Geometry of Hydrogen Dimers

Abstract

The simplest molecular dimer, H2-H2, poses a challenge to both experiment and theory as a system with a multidimensional energy surface that supports only a single weakly bound quantum state. Here, we provide a direct experimental image of the structure of hydrogen dimers [(H2)2, H2-D2, and (D2)2] obtained via femtosecond laser-induced Coulomb explosion imaging. Our results indicate that hydrogen dimers are not restricted to a particular geometry but rather occur as a mixture of all possible configurations. The measured intermolecular distance distributions were used to deduce the isotropic intermolecular potential as well as the binding energies of the dimers.

Equilibrium Thermodynamics of the Dimers and Trimers of H2 and D2 and Their Heterodimer (H2)(D2)

“…83%). This is likely an artifact caused by the characteristics of the Morse function since adjustment of the potential parameters, within the uncertainties indicated in ref 29, produces a more strongly bound J = 2 level that justifies its inclusion in the calculation of the rotational partition function. It should be noted that both the LJ and modified Buckingham potentials of ref 29 produce larger D 0 values, i.e., 5.8 and 5.4 K, respectively (c.f.…”

“…While ab initio calculations can provide values of the six harmonic frequencies and the rotational constants, it is not clear the extent to which these rovibrational levels actually contribute to the partition function because of the shallowness of the potential wells and the high zero point energies relative to the well depths, i.e., small D 0 values. However, by collapsing the six-dimensional PE surface of the dimer into a one-dimensional centrosymmetric one (e.g., the Morse potential), as is accomplished in the work of Khan et al, 29 the opportunity arises to calculate q Drot-vib since, in this model, there is only one vibrational mode, the van der Waals stretch, thus considerably simplifying the calculation. The rotational energy levels shown in Table 4 are used to calculate q Drot .…”

Small Helium Clusters Studied by Coulomb Explosion Imaging

“…Although, the FNRA has been successfully applied for obtaining ground state probability distributions of variety of small atomic and molecular clusters [14,15,20], including helium clusters [6,21,22], it was found to be inaccurate in imaging excited vibrational states of the H + 2 cation [11]. The reason is two-fold: 1) the electronic and nuclear degrees of freedom are not fully decoupled in this system and 2) the local kinetic energy (kinetic energy at a given R) can be comparable to the 1/R potential energy.…”

Perspective: Ultrafast Imaging of Molecular Dynamics Using Ultrafast Low-Frequency Lasers, X-ray Free Electron Laser and Electron Pulses

“…The Coulomb explosion imaging method provides instantaneous 3D imaging of nuclear positions [54], such as the structure of hydrogen dimers [55], laser-induced isomerization in acetonitrile (CH 3 CN) [56] and photodissociation dynamics of chlorocarbonylsulfenyl chloride [27] and CH 2 BrI [57]. In CEI experiments, the bonding electrons of molecules are stripped away by ultrashort lasers in a time scale shorter than nuclear motion, leaving positive ions in the positions of molecular nuclei.…”