The potential of ultrashort laser pulses in the laser mass spectrometry of
photounstable molecules is demonstrated for the case of nitrotoluenes. Nitr
o compounds tend to quickly dissociate after photoexcitation producing only
unspecific fragments under conventional nanosecond multiphoton ionization
conditions. The mass spectra of two isomers of mononitrotoluene, two isomer
s of dinitrotoluene and trinitrotoluene were recorded following multiphoton
ionization with 170 fs laser pulses either with a wavelength of 412 or 206
nm. Although even these laser mass spectra are characterized by intense fr
agmentation they exhibit a clear molecular ion or OH loss signals depending
on the substitution positions. Although the two mononitrotoluenes can be d
istinguished by their mass spectra at both wavelengths the two isomers of d
initrotoluene investigated show characteristic features which allow their c
lear differentiation only at 412 nm. (Int J Mass Spectrom 206 (2001) 245-25
0) (C) 2001 Elsevier Science B.V.