Photodetachment of ColdOH−in a Multipole Ion Trap

Abstract

The absolute photodetachment cross section of OH − anions at a rotational and translational temperature of 170 K is determined by measuring the detachment-induced decay rate of the anions in a multipole radio-frequency ion trap. In comparison with previous results, the obtained cross section shows the importance of the initial rotational state distribution. Using a tomography scan of the photodetachment laser through the trapped ion cloud, the derived cross section is model-independent and thus features a small systematic uncertainty. The tomography also yields the column density of the OH − anions in the 22-pole ion trap in good agreement with the expected trapping potential of a large field free region bound by steep potential walls.PACS numbers: 33.80. Eh,33.80.Ps,33.70.Ca,95.30.Ky Photodetachment of the excess electron from a negative ion represents a fundamental light-matter interaction process that reveals detailed information on atomic and molecular structure and dynamics. The additional electron is only bound by virtue of electron-electron interactions and its binding energy often depends sensitively on electron correlations in the entangled multi-electron wavefunction. Spectroscopic information, obtained from photoelectron energy measurements, is used to study the electronic, vibrational and rotational Eigenstate spectrum of anionic and neutral molecules. This also yields insight in transition states structures of reactive collisions of neutral molecules [1]. In addition, ultrafast timeresolved studies of wavepacket or electron rearrangement dynamics inside clusters have employed anion photoelectron spectroscopy [2]. Multiphoton electron detachment in short laser pulses provides information on the electronatom interaction in strong laser fields [3]. Cross sections for photodetachment reveal complementary information to photoelectron spectra and serve to challenge calculations for bound-free transition matrix elements [4].The most detailed photodetachment cross section studies on a molecular anion have been carried out for the hydroxyl anion OH − . Much of the work on OH − has focused on relative cross sections. Near threshold, comparison of the relative cross section as a function of electron energy with Wigner threshold laws allows for precise tests of the long-range electron-neutral interactions. Specifically, the subtle coupling of the two lowest Λ-doublet states has been observed, which leads to a long-range electron-dipole interaction [5,6]. Also, relative cross section measurements for transitions of specific rotational states have been carried out for OH − [7,8]. These results indicate deviations of the photodetachment cross section from s-wave electron detachment calculations. Interpretation, however, is obscured by the incomplete knowledge of the OH − rotational population. Even for the OH − anion only two absolute cross section measurements have been performed to date [9,10], which disagree with each other and deviate from the calculated one by almost an order of magnitude [4].An important ...

Absolute photodetachment cross section measurements of the O− and OH− anion

“…13 and thereby somewhat reconciles the latter two values. With our previous value, 11 which had a much larger uncertainty due to the employed one-dimensional tomography, we also find fair agreement. The data for different temperatures prove that within the error bar the cross section stays constant in the temperature range from 8 to 300 K ͑see Fig.…”

“…11 In Ref. 11 this integral was derived from a one-dimensional scan of the laser through the trap, relying on cylindrical symmetry for the ion density distribution.…”

Perspective: Electrospray photoelectron spectroscopy: From multiply-charged anions to ultracold anions

“…High-resolution PE imaging of the cold fullerene C 60 − anion Fig. 6 shows a comparison of the PE spectra of C 60 − using the first-and second-generation ESI-PES apparatuses, 42,118 and the new PE imaging data on the third-generation apparatus.…”

“…6(a)), but the spectrum was limited by the resolution of the magnetic-bottle analyzer. An extensive set of PE images were taken for C 60 − on the third-generation apparatus at different detachment wavelengths. 119 Figs.…”

Multipole electrodynamic ion trap geometries for microparticle confinement under standard ambient temperature and pressure conditions

“…Multipole traps are used as tools in analytical chemistry 69,70 . Octupole RF traps are used as buffer zones or guides in order to transport ions from one region to another in similar applications.…”