2B15

Decomposition of N-methylaniline and N,N-dimethylaniline
in intense laser fields(Univ. of Tokyo, School of Sci.) ○Peng Liu, Ryuji Itakura, Kaoru Yamanouchi

Introduction
From the series of our recent studies on the behavior of simple aromatic cation
molecules in intense fs and ns laser fields by tandem-type time-of-flight (TOF) mass
spectrometry [1-4], their decomposition and intra-cluster reaction processes were foundto be sensitively dependent on the characteristics of the laser field such as intensity andwavelength. In the present study, the responses of N-methylaniline (NMA) and N,
N-dimethylaniline (DMA) to intense ns and fs laser fields are investigated in order toreveal the effect of the methyl group substitution on the fragmentation anddecomposition processes of the aniline moiety.
Experiment
The vapor of NMA or DMA sample gas seeded in He expands into the vacuum
chamber through a pulsed valve with the stagnation pressure of 1 atm and is skimmed
by a skimmer. The fourth harmonics (266 nm) of Nd:YAG laser light crossing withthe collimated molecular beam at right angles is focused on the molecular beam toproduce the parent ions through multiphoton ionization. The fragment ions are also
produced at the photoionization stage, but only the parent ions are allowed to passthrough the first stage of the tandem-type time-of-flight (TOF) mass spectrometerequipped with a mass gate. The parent ions are irradiated with the intense femto
second (fs) laser field (λ = 395 nm, ∆t = 50 fs, I ∼ 1016 W/cm2) or with the nano second(ns) laser field (λ = 532 nm, ∆t = 6 fs, I ∼ 1010 W/cm2), and the resultant fragment
cations are mass-selected in the second stage of the tandem-type TOF massspectrometer, and are detected by a MCP detector.
Results and discussion
In Fig.1(a), the TOF mass spectrum is obtained when NMA+ is irradiated with theintense fs laser light. The major peaks are assigned to C6H5N+, C6H5+, C5H5+, C4H3+,
and C3H3+. Among them C6H5+ has the largest yield, indicating that the breaking of
the C-N bond, C6H5NHCH3+ → C6H5+ + NHCH3, is the dominant channel. In the nslaser case, basically the same ion species are seen in the TOF mass spectrum as shownin Fig.1(b). It can be noticed that the yield of C4H3+ is the largest among the smaller
ion species than C6H5+. When the laser field intensity is increased from 0.15×1010
W/cm2 to 1.38×1010 W/cm2, the relative yield of C6H5+ decreases from 76.2% to 52.2%
monotonically, while that of C4H3+ increases from 4.0% to 21.9%, and C3H3+ increases
from 2.1% to 6.3%. It is noteworthy that the sum of the yields of these three fragment
ion species is kept almost constant at 80~82 % in the entire range of the laser field
intensity, and that the yield of C5H5+ (~15%) is almost unchanged. This indicates thatC6H5+ produced in the low laser-field intensity regime as one of the nascent fragment
ion species absorbs more photons and decomposes into C4H3+ and C3H3+, while C5H5+
does not absorb additional photons. Considering this laser-field intensity dependence,Fig 1. Decomposition of N-methylaniline in (a) fs laser field (1.0×1016
W/cm2) and (b) ns laser field (1.4×1010 W/cm2).
the larger yield of C6H5+ in the fs laser field may be interpreted in a way that thetemporal duration of the laser field is too short for the nascent C6H5+ to absorb theenergy from the light field.
The fragmentation patterns of DMA+ in the fs and ns laser fields were found to be
similar to those of NMA+ except for the appearance of C6H5NCH+. In the fs laser field,
C5H4+ and C5H3+ were observed in addition to C5H5+, but C5H4+ and C5H3+ were notproduced in the ns laser field, suggesting that C5H4+ and C5H3+, which are moreunsaturated than C5H5+, could absorb additional photons more efficiently. The
mechanism of the formation of the five-member ring cations is discussed by referring tothe decomposition process of aniline cations and aniline ammonia cluster cations.
References[1] R.Itakura, J.Watanabe, A.Hishikawa, and K.Yamanouchi, J. Chem. Phys., 114, 5598
(2001).
[2] J.Watanabe, R.Itakura, A.Hishikawa, and K.Yamanouchi, J. Chem. Phys., 116, 9697
(2002).
[3] R.Itakura, T.Asano, K.Yamanouchi, J. Photochem. Photobiol. A 158, 77 (2003).
[4] R.Itakura, K.Yamanouchi, T.Yasuike, K.Someda, submitted to Chem. Phys. Lett..