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

Abstract

Absolute total photodetachment cross sections of O− and OH− anions stored in a multipole radio frequency trap have been measured using a novel laser depletion tomography method. For OH− the total cross sections of 8.5(1)stat(3)syst and 8.1(1)stat(7)syst×10−18 cm2, measured at 662 and 632 nm, respectively, were found constant in the temperature range of 8–300 K. The O− cross sections 5.9(1)stat(2)syst and 6.3(1)stat(2)syst×10−18 cm2 measured at 170 K at 662 and 532 nm, respectively, agree within error estimations with preceding experiments and increase the accuracy of the widely used calibration standard for relative photodetachment measurements of diverse atomic and molecular species.

Internal state thermometry of cold trapped molecular anions

“…By coupling the laser light through a single-mode optical fibre temperature-induced drifts of the beam pointing are suppressed. Therefore, the laser always illuminates the same spot within the ion cloud, and a full tomographic measurement 14,15,22 at every laser wavelength is not required for the relative cross section measurements.…”

“…The inset in Fig. 2 shows a twodimensional tomography scan of the spatial density of the trapped ion cloud, which is used in the determination of absolute photodetachment cross sections 14,15 .…”

How can a 22-pole ion trap exhibit ten local minima in the effective potential?

“…Its central element is a 22-pole trap [2] with good optical access along the trap axis. In the present tomography experiments, we have used an ensemble of stored OH − anions cooled to a translational temperature of either 170 or 300 K. The use of negative ions allows us to perform photodetachment measurements, were the anions are depleted by a laser beam propagating parallel to the symmetry axis of the trap [19,20]. The photodetachment process…”

“…If k(x, y) is measured for the whole ion distribution, also the absolute photodetachment cross section can be obtained [20].…”

“…[20]. A free-running continuous wave diode laser at 661.9 nm (Mitsubishi ML1J27, 100mW, spectral width 0.7 nm FWHM) has been employed.…”