Transcript Type title here

The role of (n/)* states in molecular
photodissociation processes
Mike Ashfold
University of Bristol
http://www.bristoldynamics.com
Leiden Observatory Workshop: Photodissociation in Astrochemistry
Kasteel Oud Poelgeest, Leiden
3-5 February 2015
Plan for lecture
•
Short introduction to (n/)* states
•
O–H/S–H bond fission in H2O, H2S, alcohols/thiols, etc
•
N–H bond fission in ammonia, amines
•
C–H bond fission in methane, ethyne and HCN
•
Larger molecules?
•
(n/)* state mediated ring opening?
•
Conclusions and future prospects
Excited state photochemistry: (n/π)* excited states
• HF
• Archetypal * n/ excitation.
• Repulsive excited state
potential.
• Direct bond fission
 H + F atoms
H2O (singlet state potentials shown only)
conical
intersection
• OH not spherically symmetric, presents p and p orbitals.
• 1 and 1 potentials cross at linear geometry.
• 1A components avoid each other when bent  conical intersection (CI)
at HOH = 180 and extended RO–H.
• Change in HOH with O–H bond extension  OH product rotation?
How to test such predictions?
In case of H2O:
• simple triatomic, light atoms, high I.P.
• Experiment:
photofragment translational spectroscopy (PTS) / imaging.
• Theory:
ab initio full-dimensional PESs, propagate wavepackets.
What are key wavelengths to study? Experimentalists prefer 
> 200 nm or  = 121.6 nm, but almost any wavelength is
possible if the problem merits it.
Absorption cross-sections (; T) generally not available  an
issue for light molecules with structured Rydberg regions.
H Rydberg atom PTS {Karl Welge (Bielefeld)}
Detector
Cation
Rydberg State
(H*)
n=2
Photolysis
Tagging (366nm)
Lyman-
(121.6nm)
H*
n=1
Hydrogen Atom
Molecular Beam
Lyman-
(121.6nm)
“High n” tagging
(366nm)
H2O + h (=121.6 nm)  H + OH(X/A, v, N)
 Product recoil anisotropy, electronic branching in products, immune
to effects of OH predissociation, confirm massive OH product rotation.
(see also Dr Kaijun Yuan presentation, Wed 4 pm).
Yuan et al., PNAS 2008 105 19148 (Mordaunt et al., JCP 1994 100 7360)
Hydrides (and halides)
Similar ideas go a long way to explaining/predicting photoinduced
excited state bond fission in all gas phase hydride molecules:
H2O  CH3OH, C6H5OH, … H2S, CH3SH, C6H5SH, ……
NH3  CH3NH2, cyclic amines (pyrroline, morpholine, etc),
heterocycles (azoles, indoles, adenine, etc), C6H5NH2, …..
HCN, HCCH, etc
alkylated analogues (e.g. ethers, thioethers, secondary amines, etc)
(PCCP 2010 12 1218)
families of halides
(e.g. hydrogen halides  alkyl halides, aryl halides, halophenols, ..)
(PCCP 2011 13 8075; JCP 2013 138 164318)
H2S
Similarities (but also differences) with H2O.
I.P.(H2S) < I.P.(H2O), D0(H–SH) < D0(H–OH)  observe
given photodissociation behaviour at longer  in H2S.
 = 243.3 nm
Wilson et al., Mol Phys 1996 88 841
Near UV photolysis
* 3px(HOMO) continuum
spanning 190-250 nm.
H + SH(X) products formed
predominantly in v = 0, low N
states
Anisotropic recoil
Similar behaviour to that
shown by H2O in wavelength
range 150 <  < 190 nm.
H2S + h (=121.6 nm)  H + ?
Excite just below 1st I.P. –
high density of states.
Populate (or couple to)
second n* state.
Dissociate to H + SH(A) with:
v  5 (and low N), and
v = 0 with high N.
No H + SH(X) products.
Dissociating molecules fail to
sample relevant CI in RH–SH
at linear geometries.
3-body fragmentation
 H + H + S.
H2 + S yields? (Mingli Niu
presentation, Thurs 11 am)
Cook et al., J Chem Phys 2001 114 1672
VUV photolysis of alkyl alcohols and thiols?
• HRA-PTS studies of MeOH, EtOH photolysis at  = 157.6 nm.*
• Fast H atoms from H–OMe, H–OEt bond fission on n* PES;
slower H atoms attributed to primary C–H bond fission and to
secondary decay of vibrationally ‘hot’ OMe and OEt products.
• MeSH studied at  = 193.3 nm (and longer wavelengths)
(Butler, Wittig, ourselves, Yang, Parker, ….).
H–SMe and HS–Me bond fissions studied in some detail.
• 121.6 nm photolysis of such larger polyatomic systems rarely
studied in a quantitative manner. In many cases, photoexcitation
would project molecule above first I.P., myriad fragmentation
pathways (in principle), not that appealing to photodissociation
dynamicists.
* Yuan et al, Chin. J. Chem. Phys. 2008 21 301
NH3 + h  H + ?
• 3sn excitation gives structured
A – X absorption band centred at
 ~200 nm, dominated by
progression in excited state
umbrella-bend vibration.
• Conical intersection between
ground and first excited PESs in
RH2N–H dissociation coordinate,
at planar geometries.
• Upon dissociation, parent out-ofplane vibrational motion maps
into a-axis rotation of NH2
fragments.
Mordaunt et al., J Chem Phys 1996 104 6460
NH3 + h ( = 216 nm)  H + NH2(X)
• Excess energy channelled into
product translation and rotation
• Broadly similar behaviour seen
at all wavelengths   193 nm.
• NH2(A) products also identified
once above relevant energy
threshold.
• Similar studies of NH2D, NHD2
and ND3 photolysis at these
near UV wavelengths.
• No similar quantitative study at 
= 121.6 nm (above I.P.)
• MeNH2: Me–NH2 and MeNH–H bond fission following near UV
excitation, but nothing quantitative at shorter wavelengths.
Mordaunt et al., J Chem Phys 1996 104 6460
HCN + h ( = 121.6 nm)  ?
• H + CN(A) products dominate,
(HCN = 180 )
bimodal rotational state population
distribution.
• No H + CN(X) products identified.
• Fully consistent with dissociation via * PES. Predict same for
HC2nCN, given same X2 vs A2 ordering in C2nCN radicals.
Cook et al., J Chem Phys 2000 113 994
C2H2 + h  ?
~210 nm:
• Excite (bent) valence states,
• ‘Slow’ dissociation (ISC) via
triplet states  H + C2H(X)
products.
• Beautifully quantum state
resolved problem.
121.6 nm:
• Region of high state density,
• Efficient coupling to * PES
• Dissociate to H + C2H(A) products,
with obvious activity in C=C stretch
mode.
Mordaunt et al., J Chem Phys 1998 108 519;
Loeffler et al., J Chem Phys 1998 109 5231
CH4 + h ( = 121.6 nm)  ?
A long standing challenge.
• CH4 only absorbs at  137 nm.
• H + CH3 identified as major
primary products when exciting
at 121.6 nm as long ago as
1993.
• Also see slow H atoms from
three-body dissociation.
• Mechanism?
• H atoms show speed dependent
recoil anisotropy.
(Wang et al., J. Chem. Phys. 2000 113 4146).
Mordaunt et al. 1993 98 2054
CH4 + h ( = 121.6 nm)  ?
Recent clarifications.
• Experiments at ~130 nm
(Zhang et al., J. Phys. Chem.
Letts. 2010 1 475.
• Structure in TKER spectrum
confirms H + CH3 products;
latter carry high N (and v)
excitation.
• Theory (van Harrevelt, J. Chem. Phys. 2006 125 124302)
• Identifies conical intersections between S1 and S0 PESs at planar
geometries that offer potential routes to the observed fragmentation
products.
Summary
• Focus of talk – photodissociation dynamics of hydride molecules,
using H (Rydberg) tagging methods.
• Ion imaging methods applicable to many other small fragments.
• In almost all cases, level of study (and understanding) much better
for near UV wavelengths than at  = 121.6 nm.
• (n/)* PESs enable excited state photofragmentation.
Radiationless transfer to S0 PES, and unimolecular decay of
vibrationally ‘hot’ S0 molecules becomes ever more important for
larger polyatomic molecules.
• Outstanding issues – for experiment and theory:
identification of all products
product branching ratios
(T) dependence of total (and partial for forming possible products)
Acknowledgements
Bielefeld: Karl Welge, Ludger Schnieder, Eckart Wrede (Bielefeld)
Bristol:
PhD students: Greg Morley, David Mordaunt, Steve Wilson, Claire Reed,
Phil Cook, Brid Cronin, Mike Nix, Adam Devine, Graeme King, Tom Oliver,
Tolga Karsili, Barbara Marchetti, Rebecca Ingle.
PDRAs: Ian Lambert, Steve Langford, Emma Feltham.
Academic colleagues: Richard Dixon, Colin Western, Andrew Orr-Ewing.