relaxation mechanism or the luminescence quantum yield is independent of wavelength, and the ions dissociate on the time scale of the experiment. Another issue to
be concerned about is radiative cooling by emission of infrared photons; this occurs
on the milliseconds time scale and therefore can be a problem for long dissociation
times as some ions may cool down before they dissociate. The experiment will then
provide a too low absorption-cross section.
That an ion will not fragment within the time scale of the experiment can be
due to its large size (large s). This is illustrated for myoglobin protein cations
based on work by Antoine and Dugourd [11] (Fig. 3.8). There, no fragmentation
was observed after irradiation by 260 nm as the change in internal energy after
1000
1200
1400
1600
1800
2000
0.0
0.2
0.4
0.6
0.8
1.0
m/z
10
-
Photofragmentation yield (arb. units)
Wavelength (nm)
Ion intensity (arb. units)
225
250
275
300
325
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0.0
0.2
0.4
0.6
0.8
1.0
1.2
11
-
11
+
c
b
a
Fig. 3.8 Photodissociation
(260 nm) mass spectra of apomyoglobin (a) 11
+ cations
and (b) 11
À anions. (c) The
action spectrum of the anions
obtained by measuring
the production of 10
À•
anions [11]
28
J.A. Wyer