1540
J. Chem. Phys., Vol. 110, No. 3, 15 January 1999
Luckhaus, Scott, and Crim
ergy Sciences of the U.S. Department of Energy. D.L. ac-
knowledges financial support during part of this work
through a postdoctoral fellowship of the Deutsche Fors-
chungsgemeinschaft.
major contribution from rotational excitation of NH radicals
2
is unlikely, since the lowest N–O dissociation channel cor-
relates with the excitation of an O lone pair electron, not of a
N lone pair electron.
The OH radicals mostly arise in their vibrational ground
state, although the fraction of OH in the first vibrationally
excited state increases with the parent excitation of OH
stretch overtones in the vibrationally mediated photodisso-
ciation ͑8% for excitation via 5OH). The average rotational
1
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Ϫ1
pendent of the available energy, but varying by Ϯ270 cm
for different parent vibrational excitation. As a consequence
of the torsional excitation, which accompanies the electronic
excitation, the single-photon dissociation experiments show
a preference for the AЉ component of the OH ⌳ doublets and
for the parallel alignment of the OH rotational angular mo-
mentum relative to the electronic transition dipole. We ex-
plain the loss of both preferences in the vibrational by me-
diated photodissociation with the increasing stretch–bend
5
6
7
8
9
0
1
23,40
coupling in high O–H stretch overtones of NH OH.
2
11
The fragment recoil energy accounts for about half the
Ϫ1
12
2
0 550 cm available at 240 nm ͑less in the vibrationally
mediated photodissociation͒ but takes an increasing share at
1
1
3
4
Ϫ1
lower excitation energies ͑82% of 5820 cm ͒ with the in-
ternal energy of NH decreasing correspondingly.
2
1
1
1
5
6
7
We have analyzed our findings on the basis of potential
energy surfaces calculated ab initio for the lowest singlet
states of NH OH. In particular we have been able to explain
2
the differences between our results and those from the pho-
18
tolysis at 193 nm reported by Gericke et al.27 The crossing
19
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20
21
close to the Franck–Condon region appears to dominate the
photofragmentation dynamics. As a result of this crossing the
͑
1995͒.
22
͑adiabatic͒ S1 surface shows a barrier of around 0.5 eV,
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6
which separates a bound region of the potential correlating
23
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2
˜
24
with NH (A A ) from a directly dissociative part correlat-
2
1
ing with both fragments in their electronic ground state. This
2
2
5
6
picture also explains the preferential production of H atoms
observed by Betts et al.25 in direct compared with Hg-
27
sensitized photodissociation of NH OH. The observed dif-
2
2
2
8
9
ferences simply arise from the different excitation wave-
lengths used rather than from different excitation
mechanisms: The smaller wavelengths ͑ϳ200 nm͒ used for
the direct photolysis favor H atom elimination while the
N–O dissociation channel dominates at the smaller excita-
tion energies ͑254 nm͒ used in the Hg-sensitized photofrag-
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mentation. The Hg–NH OH energy transfer might only play
2
3
3
3
3
4
5
6
7
a minor role.
Among the open questions remain details of the product
distributions and in particular the absence of velocity anisot-
ropy and rotational helicity in our experiments. More de-
tailed experiments including dispersed fluorescence detection
3
3
4
8
9
0
V. Staemmler, Acta Phys. Pol. A 74, 331 ͑1988͒.
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of NH together with quantum mechanical modeling of prod-
2
uct distributions could address these questions.
41
42
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1985͒.
ACKNOWLEDGMENTS
43
44
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7 ͑1962͒.
9
We gratefully acknowledge the support of this research
by the Division of Chemical Sciences, Office of Basic En-
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͑1979͒.
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