Absolute absorption cross section cell

A windowless absorption cell is available and can be inserted at the back of the experimental stations, thus, during another kind of measurement, the user can measure simultaneously his /her absorption spectrum, which gives a very precise calibration and a check of the monochromator resolution, and an I0 spectrum for normalisation to the absolute flux.

The absorption cell has been designed to consent the acquisition of absolute cross section spectra. It is a window-less, double ionization chamber device similar to that described by Samson et al. (1994). It is located behind another experimental vacuum chamber equipped with a 1000 l/sec turbopump, and additionally the beamline contains a differential ion pump that can sustain a pressure differential of 5 orders of magnitude. However typical pressure inside the cell is below 10 mbar to avoid non-linear effects in the absorption process. The pressure is measured by a calibrated Baratron, model 121 AA-00001C, MKS. The ambient temperature in the experimental hall is tipically 295 K and ideal behaviour can be assumed in the calculation of the gas density.

The double ionization section consists of a tube 300 mm long and with an inner diameter of 35 mm to which it is possible to apply a voltage V from 0 to ±500 V. The charged particles are collected at two identical electrodes 143 mm long and measured by two model 487 Keithley picoamperometers.

The present desing of the cell is much symmetric as it is possible, to avoid termination errors in the reading of the currents.

Indeed, in a first time it was found that at low gas pressure, when the currents should have been equal (negligible absorption), the second cell read a slightly higher current than the first cell. This error was found to be due to photoelectrons from a photodiode behind the cell, now removed. Itchkawitz et al (1995) discussed the effect of higher order light contamination for the case of a single chamber gas cell. They showed that higher order radiation causes an apparent reduction of the cross-section as well as increasing and distorting the background. A double chamber gas cell is much less sensitive, but still shows some effects, mainly associated with the continuum background rather than the absorption peak.

The gas cell gives results within 8 % of literature values for He in the continuos part of the spectrum

Selected publications

1. "Measurement and ab initio calculation of the Ne photoabsorption spectrum in the region of the K-edge", M. Coreno, L. Avaldi, R. Camilloni, K. C. Prince, M. de Simone, R. Colle, S. Simonucci, Phys. Rev. A 59 (1999) 2494.
2.  "Inner shell excitation spectroscopy of the tetrahedral molecules CX4 (X = H, F, Cl)", M. de Simone, M. Coreno, M. Alagia, R. Richter and K.C. Prince, J. Phys. B: At. Mol. Opt. Phys. 35 (2002) 61.
3. " High Resolution Inner-Shell Spectroscopy and Ab-Initio CI Calculations on TiCl4 and Isolectronic Molecules" M. de Simone, G. Fronzoni, P. Decleva, M. oreno, K. C. Prince, P. Franceschi, S. Furlan, C. Furlani; Phys. Chem. Chem. Phys. 5 (2003) 2758.
4.  “X-Ray absorption spectroscopy of VOCl3, CrO2Cl2 and MnO3Cl: an experimental and theoretical study”, G. Fronzoni, M. Stener, P. Decleva, M. de Simone, M. Coreno, P. Franceschi, C. Furlani, and K.C. Prince, J. Phys. Chem.A 113 (2009)2914–2925. DOI:10.1021/jp808720z
5.  “The C 1s and N 1s near edge x-ray absorption fine structure spectra of five azabenzenes in the gas phase”,G. Vall-llosera, B. Gao, A. Kivimaki, M. Coreno, J. Alvarez Ruiz, M. de Simone, H. Ågren, and E. Rachlew, J. Chem. Phys. 128 (2008) 044316.