The "Classical" Collinear Laser Spectroscopy

All methods used in the COLLAPS experiment are based on the technique of the collinear laser spectroscopy. This method was developed in mid 1970-ties by S.L. Kaufman (see S.L. Kaufman: "High resolution laser spectroscopy in fast beams"; Optics Communications Vol.: 17, No.: 3 (June 1976); P.: 309-312). The basic principle is a collinear superposition of a fast beam of atoms or ions with a laser beam. By scanning the laser in the rest frame of the atoms / ions the frequency can be tuned to the resonance frequency of the electronic transition in the atomic shell. After being excited by the laser, the atoms decay by emitting photons isotropically into space. This fluorescence light can be detected if photon-detection system is placed perpendicularly to the direction of the laser / atom beam.

Narrow line widths

The collinear laser spectroscopy offers an advantage which is an result of accelerating ions from an ion source: The optical line widths, which are Doppler broadened (e.g. in gas cells up to several GHz) are reduced almost to the natural line width of the specific transition under investigation (typically in the range of 100 MHz).

 

Energy spread of atoms in an ion source remains constant when they are accelerated

Reduction of line width with increasing kinetic energy / acceleration voltage

dE = d(1/2 mv2) = mv*dv =

(mc2/n2)DnD*dnD = const

 

DnD(U) = 1/2(kT/eU)1/2DnD(U=0)

Where:

dn = Doppler shift

DnD = Doppler line width

 

Example:

Neon in a ISOLDE plasma source, U=60 kV:

T(source) = 2000 K

DwD(U=0) = 3.49 GHz

DwD (U=60kV) = 2.96 MHz

Post-acceleration of the ions

The advantage of the intrinsically narrow line width of optical lines is supported by the advantage of reaccelerating (retarding) the incoming ions in the COLLAPS setup. The reacceleration is done by applying a sweepable voltage to a set of retardation lenses. The reacceleration of the ions causes a frequency sweep of the laser frequency in the rest frame of the atoms. This method assures narrow laser line widths, because the laser can be used in fixed frequency mode and doesn't have to be scanned (the latter can cause instabilities and line broadening of the laser).

Ion neutralization

Laser spectroscopy is (more or less) limited to the visible electromagnetic spectrum, because even modern spectroscopy lasers are limited to this wavelength region without using sophisticated frequency doubling techniques. This is the reason why the need of discharging the ions comes up. Because of their lower binding energies of electrons, transitions in the atomic system have lower excitation energies than in ions. In our experiment the charge exchange is done by collisional charge transfer from alkaline atoms in an alkaline metal vapor, which is produced in the charge exchange cell. However, we sometimes do spectroscopy on ions, not atoms. Two examples are: previous measurements on beryllium (Be+) ions and a planned experiment, where magnesium ions (Mg+) will be investigated.

Collinear laser spectroscopy and ISOLDE

The collinear laser spectroscopy is an ideal tool to be used in an on-line experiment at a radioactive beam facility. The collinear laser spectroscopy setup can be placed directly in the beam line of the beam facility. All measurements can be done in flight, which is the optimum method for the investigation of very short lived radioactive isotopes. The on-line capability together with the very reliable and variable on-line facility ISOLDE (Isotope Separator On-Line DEvice) at the European Laboratory for Nuclear Research CERN were the major ingredients for a quite successful tradition of on-line laser spectroscopy experiments on short lived isotopes of a large variety of chemical elements.

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