New method to study ion-molecule reactions at low temperatures and application to the H${}_2^{+}$ + H${}_2$ $\to$ H${}_3^{+}$ + H reaction

Abstract

Studies of ion-molecule reactions at low temperatures are difficult because stray electric fields in the reaction volume affect the kinetic energy of charged reaction partners. We describe a new experimental approach to study ion-molecule reactions at low temperatures and present, as example, a measurement of the ${\mathrm{H}}_2^{+}+{\mathrm{H}}_2\to {\mathrm{H}}_3^{+}+\mathrm{H}$ reaction with the ${\mathrm{H}}_2^{+}$ ion prepared in a single rovibrational state at collision energies in the range $E_{\mathrm{col}}/k_{\mathrm{B}}=5$-60 K. To reach such low collision energies, we use a merged-beam approach and observe the reaction within the orbit of a Rydberg electron, which shields the ions from stray fields. The first beam is a supersonic beam of pure ground-state H${}_2$ molecules and the second is a supersonic beam of H${}_2$ molecules excited to Rydberg-Stark states of principal quantum number $n$ selected in the range 20-40. Initially, the two beams propagate along axes separated by an angle of 10${}^{\circ }$. To merge the two beams, the Rydberg molecules in the latter beam are deflected using a surface-electrode Rydberg-Stark deflector. The collision energies of the merged beams are determined by measuring the velocity distributions of the two beams and they are adjusted by changing the temperature of the pulsed valve used to generate the ground-state ${\mathrm{H}}_2$ beam and by adapting the electric-potential functions to the electrodes of the deflector. The collision energy is varied down to below $E_{\mathrm{col}}/k_{\mathrm{B}}=10$ K, i.e., below $E_{\mathrm{col}}\approx 1$ meV, with an energy resolution of 100 $\mu$eV. We demonstrate that the Rydberg electron acts as a spectator and does not affect the cross sections, which are found to closely follow a classical-Langevin-capture model in the collision-energy range investigated. Because all neutral atoms and molecules can be excited to Rydberg states, this method of studying

39 pages, 10 figures

References in corpus (8)

• Observation of Quantum Effects in sub Kelvin Cold Reactions
• Reaction Networks For Interstellar Chemical Modelling: Improvements and Challenges
• Radiofrequency multipole traps: Tools for spectroscopy and dynamics of cold molecular ions
• Dynamics of gas phase Ne${}^{\ast }$ + NH${}_3$ and Ne${}^{\ast }$ + ND${}_3$ Penning ionization at low temperatures
• Observation of orbiting resonances in He(3S1) + NH3 Penning ionization
• Quantum Langevin model for exoergic ion-molecule reactions and inelastic processes
• Radiative and collisional processes in translationally cold samples of hydrogen Rydberg atoms studied in an electrostatic trap
• Simulating rotationally inelastic collisions using a Direct Simulation Monte Carlo method

Cited by in corpus (22)

• The Central 300 pc of the Galaxy probed by infrared spectra of H${}_3^{+}$ and CO: part I. Predominance of warm and diffuse gas and high H${}_2$ ionization rate
• Collisions between cold molecules in a superconducting magnetic trap
• Observation of enhanced rate coefficients in the H${}_2^{+}$ + H${}_2$ $\to$ H${}_3^{+}$ + H reaction at low collision energies
• Reaction blockading in charged-neutral excited-state chemistry at low collision energy
• Ion-molecule reactions below 1~K: Observation of a strong enhancement of the reaction rate of the ion-dipole reaction He${}^{+}$+ CH${}_3$F
• Multipole-moment effects in ion-molecule reactions at low temperatures: part I -- Ion-dipole enhancement of the rate coefficients of the He${}^{+}$ + NH${}_3$ and He${}^{+}$ + ND${}_3$ reactions at collision energies near $0$ K
• Electrically tuned Förster resonances in collisions of NH${}_3$ with Rydberg He atoms
• Cold collisions in a molecular synchrotron
• Direct Observation of a Feshbach-resonance by Coincidence-detection of Ions and Electrons in Penning Ionization Collisions
• Measuring the dispersive frequency shift of a rectangular microwave cavity induced by an ensemble of Rydberg atoms
• Fluorescence-lifetime-limited trapping of Rydberg helium atoms on a chip
• Quantum-State-Specific Reaction Rate Measurements for the Photo-induced Reaction Ca${}^{+}$ + O${}_2$ $\to$ CaO${}^{+}$ + O
• Experimental demonstration of a Rydberg-atom beam splitter
• Probing resonant energy transfer in collisions of ammonia with Rydberg helium atoms by microwave spectroscopy
• Opposite effects of the rotational and translational energy on the rates of ion-molecule reactions near $0\text{K}$: the ${\text{D}}_2^{+}+{\text{NH}}_3$ and ${\text{D}}_2^{+}+{\text{ND}}_3$ reactions
• Matter-wave interferometry with atoms in high Rydberg states
• Confinement of high- and low-field-seeking Rydberg atoms using time-varying inhomogeneous electric fields
• Measurements and simulations of rate coefficients for the deuterated forms of the H2 + + H2 and H3 + + H2 reactive systems at low temperature
• Observation of quantum capture in an ion-molecule reaction
• Electrostatic trapping of N${}_2$ molecules in high Rydberg states
• Single-Source, Collinear Merged-Beam Experiment for the Study of Reactive Neutral-Neutral Collisions
• Single-atom detection with a quantum-controlled mechanical oscillator