Experimental Techniques
3
Jean Ann Wyer
Abstract
Gas-phase ion spectroscopy requires specialised apparatus, both when it comes
to measuring photon absorption and light emission (fluorescence). The reason is
much lower ion densities compared to solution-phase spectroscopy. In this
chapter different setups are described, all based on mass spectrometry and
many of them home-built: electrostatic ion storage devices, accelerator mass
spectrometers (i.e., sector instruments), reflectron time-of-flight mass
spectrometers, and ion traps. The experimental results presented in this volume
were obtained with such instruments. Detection schemes are detailed, both for
the identification of neutral products and charged ones. In delayed dissociation
experiments, prompt dissociation is a problem as all the fragmentation is then
not sampled; an example is photo-induced electron transfer to an ammonium
group and subsequent hydrogen loss. A way to circumvent this is discussed
based on a chemical approach, namely tagging of ammonium groups by crown
ether. Prompt dissociation can sometimes be identified from the total beam
depletion differing from that due to statistical dissociation. Special emphasis
in this chapter is on the limitations and pitfalls in data interpretation, and the
advantages and disadvantages of the different techniques are clarified. New
instrumental developments involving cryo-cooled storage rings, which show
great promise for the future, are briefly touched upon.
3.1
Action Spectroscopy
In conventional spectroscopy the absorbance of light by a sample is calculated
using the intensities of the incident and transmitted light according to the LambertBeer law. However, for ionic samples the density of molecules is too low to cause a
J.A. Wyer (*)
Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark
e-mail: jeanwyer@phys.au.dk
S. Brøndsted Nielsen and J.A. Wyer (eds.), Photophysics of Ionic Biochromophores,
Physical Chemistry in Action, DOI 10.1007/978-3-642-40190-9_3,
# Springer-Verlag Berlin Heidelberg 2013
21
3
Jean Ann Wyer
Abstract
Gas-phase ion spectroscopy requires specialised apparatus, both when it comes
to measuring photon absorption and light emission (fluorescence). The reason is
much lower ion densities compared to solution-phase spectroscopy. In this
chapter different setups are described, all based on mass spectrometry and
many of them home-built: electrostatic ion storage devices, accelerator mass
spectrometers (i.e., sector instruments), reflectron time-of-flight mass
spectrometers, and ion traps. The experimental results presented in this volume
were obtained with such instruments. Detection schemes are detailed, both for
the identification of neutral products and charged ones. In delayed dissociation
experiments, prompt dissociation is a problem as all the fragmentation is then
not sampled; an example is photo-induced electron transfer to an ammonium
group and subsequent hydrogen loss. A way to circumvent this is discussed
based on a chemical approach, namely tagging of ammonium groups by crown
ether. Prompt dissociation can sometimes be identified from the total beam
depletion differing from that due to statistical dissociation. Special emphasis
in this chapter is on the limitations and pitfalls in data interpretation, and the
advantages and disadvantages of the different techniques are clarified. New
instrumental developments involving cryo-cooled storage rings, which show
great promise for the future, are briefly touched upon.
3.1
Action Spectroscopy
In conventional spectroscopy the absorbance of light by a sample is calculated
using the intensities of the incident and transmitted light according to the LambertBeer law. However, for ionic samples the density of molecules is too low to cause a
J.A. Wyer (*)
Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark
e-mail: jeanwyer@phys.au.dk
S. Brøndsted Nielsen and J.A. Wyer (eds.), Photophysics of Ionic Biochromophores,
Physical Chemistry in Action, DOI 10.1007/978-3-642-40190-9_3,
# Springer-Verlag Berlin Heidelberg 2013
21