2P14
Elucidation of Photodissociation Mechanism of Methylamine by REMPI
Spectroscopy and Ion Imaging Experiment
(Hiroshima Univ.) Y. Onitsuka, Y. Kohge, K. Yamasaki, and H. Kohguchi
It is expected in analogy with ammonia that methylamine (CH3NH2) predissociates, following to UV
absorption, from the photoinitiated the nN(3s) state to the nN*C-N or nN*N-H surfaces, leading to C-N
and N-H bond cleavage. However recently, an intricate and novel dissociation mechanism to NH (X3-)
+ CH4 was reported as the primary process,[1] which was supported by the theoretical calculation.[2]
These results imply that the photodissociation of CH3NH2 are so complex that even major product
pathways remain unclear; it has been still questionable whether a simple bond cleavage (C-N or N-H
bonds) occurs. In order to reveal the predissociation mechanisms involving two bond cleavages (C-N
and N-H bonds), we have measured the final state distributions and the state-resolved scattering
distributions of the photofragments. In this study, we have achieved detections of not only NH (X3-)
reported but also CH3, the latter of which was produced by a simple C-N bond cleavage.
We have carried out the pump-probe experiment using the molecular beam of CH3NH2 at photolysis
wavelength of 213 nm (the 5th harmonics of Nd: YAG laser). The photofragments of CH3 and NH (X3-)
were selectively detected by resonantly enhanced multi-photon ionization (REMPI) spectroscopy. The
state-resolved scattering distributions of these photofragments were measured using velocity map ion
1
The observed REMPI spectra of the CH3 and NH (X3-)
fragments are shown in Figs.1 and 2, respectively. The REMPI
spectrum of CH3 showed the typical 000 band and the
characteristic 111 band. For NH (X3-), we observed the REMPI
Intensity / arb. unit
imaging technique coupled with the REMPI spectroscopy.
0
59850
distribution of CH3 (v=0) and NH (X3-, v=0) showed two and
one velocity components, respectively. These results indicate
that the direct and indirect competing processes operate in the
photodissociation of CH3NH2; two mechanisms of the C-N
bond cleavage to produce CH3, while an intricate mechanism
59950
60000
-1
Fig.1
(2+1) REMPI spectrum of the
,,
fragment CH3 (X 2A2 , v=0 and v1 =1 )
1
Intensity / arb. unit
by Thomas and coworkers.[1] The observed scattering
59900
Two-photon Wavenumber / cm
spectrum similar to the results at 222 nm and 240 nm reported
0
involving two bond cleavages (C-N and N-H bonds) produces
NH (X3-).
89100
89200
89300
-1
Three-photon Wavenumber / cm
Fig.2 (3+1) REMPI spectrum of the fragment
NH (X3-, v=0)
Reference
[1] J. O. Thomas, K. E. Lower, and C. Murray, J. Phys. Chem. Lett. 3, 1341-1345, 2012.
[2] H. Xiao, S. Maeda, and K. Morokuma, J. Phys. Chem. A, 117, 5757-5764, 2013.