A. Demchuk et al. / Chemical Physics Letters 348 /2001) 217±222
219
features at masses 87±89 amu with the most in-
tense peak being at mass 88. The fragments pro-
duced during the ionization of the parent molecule
are also shown. At masses 58 and 59 amu, one
clearly identi®es the NꢀCH3 fragment resulting
3
from cleavage of the Al±N bond, while the AlH3
fragment is represented by the peak at 30 amu.
The trimethylamine portion of the molecule also
shows daughter ions .masses 42±44 amu) resulting
from the loss of a methyl group, while the methyl
fragment itself is found at mass 15.
The mass spectrum of neat TMAA subsequent
to 193 nm laser irradiation is shown in Fig. 1b.
Alternatively, Fig. 1c presents the results in sub-
traction mode .i.e., Fig. 1a is subtracted from Fig.
1b). Note the dierent scaling factor. As shown in
Fig. 1c, laser photolysis causes the parent molecule
signal to decrease somewhat in intensity as evi-
denced by the negative peaks shown in these
spectra. With regard to the higher mass spectral
features, the 193 nm laser irradiation of the
TMAA gas pulse clearly results in clustering. One
could speculate on the mechanism behind this
photolytic process, but such discussion is beyond
the scope of this Letter. Suce it to say that sig-
ni®cant clustering is readily apparent.
Fig. 2. Mass spectra for a TMAA/NH3 gas expansion taken .a)
without laser excitation and .b, c) with 193 nm laser excitation.
For .c), the result was obtained by subtracting spectrum .a)
from spectrum .b).
H
3ÀxAlNH2Ày and associated hydrogen-loss ions.
3.2. Laser-initiated reactivity of TMAA with NH3
and ND3
While these daughter ion species can readily be
formed during the EI ionization detection event, it
is possible that the features may also partially arise
via neutral pathways involving hydrogen elimina-
tion. At present, the various pathways cannot be
distinguished unequivocally.
Mechanistically, we assert the following. The
193 nm photolysis of NH3 produces very reactive
NH2 radicals. An NH2 radical subsequently en-
counters a TMAA molecule and displaces the
Fig. 2 presents spectra analogous to those taken
for Fig. 1, the procedural dierence being the ad-
dition of NH3 through a second pulsed nozzle.
When compared with Fig. 1a, one can see in Fig.
2a that the addition of NH3 produces expected
new peaks at 16 and 17 amu. No other additional
products are detected. However, when the 193 nm
radiation is introduced, as shown in Fig. 2b, a
dramatic change in the spectrum is induced. Note
the essentially complete disappearance of the par-
ent TMAA signal. While direct photolysis of the
TMAA occurs to some extent as shown in Fig. 1, it
simply cannot explain the complete loss of TMAA
parent signal. The fate of the TMAA can be found
in the subtraction-mode spectrum of Fig. 2c. .Note
the scale change.) Here, the TMAA parent fea-
ture.s) is clearly negative, and the primary metal±
organic product feature.s) is attributable to
NꢀCH3 portion thereby forming H3AlNH2. The
3
detection process results in the spectrum shown in
Fig. 2c. A blow-up of the clustering portion of this
spectrum also reveals interesting behavior as shown
in Fig. 3b. Speci®cally, the clustering eects that are
present when neat TMAA is photolyzed .Fig. 3a)
disappear when the NH3 is added. This data along
with previous results suggest that the H3AlNH2
species is anomalously stable or at least relatively
inert in this type of clustering environment.