Ultra-Sensitive Absorption Spectroscopy on
lons using UV/VUV Synchrotron Radiation

K.L. Mullman, M. Sakai, and J. E. Lawler
University of Wisconsin-Madison
Madison, WI 53706


An absorption experiment utilizing continuum radiation from the Aladdin storage ring, a 3m focal length vacuum echelle spectrometer, and a CCD detector array is reported. The extremely high spectral radiance of the storage ring in the UV and VUV provides unique capabilities for both basic atomic spectroscopy and for plasma diagnostic work. The thinned, backilluminated, boron doped (UV sencitive) CCD array extends the usual signal to noise advantage of a detector array in an absorption experiment to short wavelengths. This experiment achieves both spectroscopic resolving powers of 350,000 and sensitivities to fractional absorptions much smaller than 1% at deep UV and VUV wavelengths. Absorption spectra relative absorption oscillator strengths for Fe+ in a high current hollow cathode are presented. This experiment is applicable to many atomic ions in a variety of discharge plasmas.

There are three main components of the experiment

  • ・The Continuum Source
  • ・The Absorbing Sample
  • ・The Spectrometer/Detector System


The continuum source is the white light beamline on the Aladdin storage ring at the Synchrotron Radiation Center.


The synchrotron Provides a bright photon beam ideal for our atomic absorption experiment in the 4-11 eV (300-110nm) range.


The absorbing source is a hollow cathode discharge

  • ● 1cm inner diameter
  • ● 10 cm long
  • ● double sided
  • ● water cooled

We use discharge currents from 0.02 A to 3.00 A, corresponding roughly to ion densities of 1010 to 1013cm-2.

The hollow cathode discharge is an ideal low-pressure gas-phase source for essentially every element in the periodic table, both neutral and singly ionized species.


Since we are measuring relative oscillator strengths, it is ideal to have two spectrometer/detector systems. This allows simultaneous measurements of the“reference”and the“unknown”,eliminating the need for long-term stability in the hollow cathode discharge.

One of our spectrometers is a high resolution instrument, the other is an intermediate resolution instrument.


We have given new life to the spectrometers by equipping them with state of the art multichannel detector arrays.

The CCD Array

The high resolution spectrometer has CCD array. It is an un-coated,borondeped, thinned, back-illuminated, deep UV sensitive device(SITe chip, Princeton Instruments hardware).CCD’s offer tremendous sensitivity with low read noise.

The Photo-diode Array(PDA)

The intermediate resolution spectrometer has PDA (EG&G chip,Princeton Instruments hardware). The PDA has a much larger photoelectron well than the CCD array. However the higher read noise requires either higher light levels or longer integration times.


This multichannel absorption experiment is superior to single channel sequentially scanned absorption experiments. In a single channel experiment, any fluctuations or flicker in the continuum source are mapped into the noise, directly limiting the sensitivity of the measurements. In a multichannel experiment such as ours, all spectral elements are measured simultaneously, making intensity fluctuations or flicker inconsequential. The sensitivity of the measurements is now limited by Poisson Statistics, which accumulate quickly in a multi-channel experiment.


The“Equivalent Width”is the integrated area of normalized absorption feature.

For the normalized lineshape, g(ν-ν0), we assume a full Voigt profile. The Lorentzian contribution is assumed to be primarily due to radiative (natural) broadening. The temperature of the atoms in the discharge is found by finding the best overall consistency in absorption measurements for several wellknown UV line pairs at each current used in the hollow cathode discharge.


We see evidence of a non-Maxwellian velocity distribution in our discharge. A model of our discharge suggests that this is due to competition between the ambipolar diffusion (ion loss) rate and the collisional thermalization that fill out the high energy tail of the Maxwellian profile. With our discharge parameters and operating conditions, the diffusive loss rate and the collisional thermalization rates are nearly the same. Thus the more energetic ions diffuse out of the negative glow to the cathode of the discharge and are lost at a higher rate than low energy ions. Collisional thermalization is unable to completely“fill out”the high energy tail of the Maxwellian distribution. This is referred to as“diffusive cooling”and results in a truncated Voigt profile.


To determine the effective ion temperature of our discharge, we measure several strong, well-known UV line pairs at a given discharge current and find the temperature which gives the best overall consistency for all of the rations. Since we continue to assume a full. Voigt profile in our curve of growth analysis despite the diffusive cooling in our discharge, the result of our temperature determination is artificially low or unphysical (below 293K, the temperature of the hollow cathode discharge cooling water) for discharge operating currents below 300mA. We recognize that this effective ion temperature is not the true heavy particle temperature of the discharge, but we use it in analyzing Fe+ absorption data with the discharge operating under a well defined set of conditions.

Well-known f-value ratios in Fe II and our measurements at each hollow cathode discharge current used in the experiment. The last row gives the effective ion temperature used for the Voigt profile in the curve of growth analysis.

aOscillator strengths from emission branching fraction and laser induced fluorescence lifetime measurements
(Bergeson, S.D., Mullman, K. L., Wickliffe, M.E., Lawler, J.E., Litzen, U., Johansson, S., 1996, ApJ 464)

Absolute VUV absorption oscillator strengths compared to values in the literature

a Nussbaumer, H., Pittini, M., Storey, P.J., 1981, A&A 102, 351
b Shull, J. M., Van Steenberg, M., Seab, C. G., 1983, ApJ 271, 408
c Kurucz, R. L., 1988, Trans. IAU 20B, 168
d Fawcett, B. C., 1988, Atomic Data and Nuc. Data and Nuc. Data Tables 40, 1
e Cardelli, J. A., Savage, B. D., 1995, ApJ 452,275
f Bergeson, S. D., Mullman, K. L., Lawler, J. E., 1996, ApJ, 464,000






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