High resolution photofragment translational spectroscopy PTS of hydride molecules following

1
High resolution photofragment translational
spectroscopy (PTS) of hydride molecules following
Lyman-? excitation  Mike Ashfold   School of
Chemistry, University of Bristol, Bristol, U.K.
BS8 1TS http//www.chm.bris.ac.uk/pt/laser/laserh
om.

• Examples from past work H2S and CH4
  photodissociation.
• Current work UV photolysis of allene and
  propyne.

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H Rydberg atom PTS. Consider a jet-cooled
sample of hydride molecules, RAH, that absorb
photons of energy Ephot and subsequently fragment
to yield H (or D) atoms and a radical co-fragment
RA.
Experimentally, we measure the time-of-flight
(TOF) spectrum of the H (or D) atom products from
their instant of creation in the interaction
region to a detector located at a known distance,
d.
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Experimental

• Supersonic molecular beam of target hydride
  molecule, seeded in Ar.
• Dissociation initiated by photolysis laser.
• H(D) atoms are tagged 10 ns later by Lyman-?
  and 366 nm laser pulses.
• TOF spectrum of H(D) atoms reaching detector is
  recorded.
• Recoil anisotropy, and photolysis laser
  wavelength and power dependence, can be
  investigated also.

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Rydberg tagging

• H(D) atoms are tagged, at source, by two-photon
  double resonant excitation to a Rydberg state
  with high principal quantum number, n.
• The resulting Rydberg atoms are
• neutral, and long-lived.
• H atoms that recoil along the
• detection axis are field-ionised
• immediately prior to detection.
• This strategy obviates the blurring
• (from space-charge effects) that
• limits the ultimate resolution of ion tagging
  methods, and the imprecision in d that limits the
  KE resolution achieved with so-called universal
  detectors (electron impact mass spectrometer).

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Measurements data analysis, I

• d is known, thus any given TOF (tH) can be
  converted into an
• H atom velocity, vH, and thus kinetic energy,
  Ek(H).
• Knowing vH and the mass of the RA co-fragment,
  momentum conservation enables determination of
  Ek(RA) and thus the total kinetic energy release.
  
• Energy conservation arguments then allow
  derivation of information on the
• - internal (electronic, vibrational, rotational)
  energy states of the RA fragment, Eint(RA),
• - population distribution within these product
  states, and
• - strength of the dissociating bond, D0(RA?H).

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Measurements data analysis, II

• The flight path d is determined by measuring TOF
  spectrum of H atoms resulting from a very well
  characterised UV photolysis, e.g.
• the 121.6 nm photolysis of H2S.
• Limitations
• - Have to make assumptions about the mass of
  partner fragment.
• - Only sensitive to dissociation pathways
  forming H or D atoms.

TKER Total Kinetic Energy Release mH mass
of H atom mRA mass of partner fragment d
length of flight path t TOF time of arrival
Eint(RA) Ephot - D0 (RA?H) - TKER

• Gain information about internal energy of
  unobserved partner fragments.

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121.6 nm photolysis of H2S and D2S
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121.6 nm photolysis of H2S and D2S
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121.6 nm photolysis of H2S and D2S
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121.6 nm photolysis of H2S and D2S

• TKER spectra reveal two fragmentation pathways
• highly structured, associated with 2-body
  dissociation to
• H2S h? ? H(2S) SH(A2?).
• The SH(A) fragments are formed in a wide range
  of v, N states.
• improved A state term values and potential energy
  function.
• D0(H?SH) ? 31430 ? 20 cm-1, D0(D?SD) ? 31874 ? 22
  cm-1.
• broad and relatively unstructured, attributable
  to
• SH(A) ? S(3P) H Predissociation
• H2S h? ? 2H(2S) S(1D). Direct 3-body
  dissociation
• Complementary ab initio electronic structure and
  classical trajectory
• calculations provide a rationale for observed
  energy disposal and
• the non-observation of any ground (X2?) state SH
  fragments.
• (P.A. Cook et al, J. Chem. Phys. 114, 1672
  (2001)).

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Some operational parameters

• Lyman-? photon fluxes photons/pulse
• Frequency tripling in Kr (60 Torr) 109
  -1010
• Phase matched freq. tripling in Kr/Ar (2 bar)
  gt1011
• 4 wave difference freq. mixing (212.5 845 nm)
  in Kr/Ar 1012-1013
• Lyman-? cross-section 3 x 10-13 cm2
• feasible to saturate probe transition.
• Bandwidth lt 0.5 cm-1 (i.e. ??/? lt 10-5 )
  depending on laser.
• Pulse duration 5-10 ns, rep. rate 10 Hz.
• Factors affecting energy resolution of PTS
  experiment
• Spread of internal energies in parent sample.
• Finite solid angle subtended by detector (blurs
  d).
• Finite time resolution (?t 10 ns) of detector.
  
• Energy resolution, ?E/E, 0.1 has been
  demonstrated.

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121.6 nm photolysis of CH4 and CD4
H atom TOF spectrum from CH4 photolysis shows
no fine structure, but its magnitude is
sensitively dependent on the relative alignment
of ?phot and the detection axis.
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121.6 nm photolysis of CH4
? depends on TKER
2HCH2
HCH3
HCHH2
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121.6 nm photolysis of CH4
A plausible decomposition of the TKER spectrum
H atoms from secondary decay of highly internally
excited CH3(X) products formed by processes I
and II.
I. Dissociation to HCH3(X) products following
excitation to 1A Jahn-Teller component of 1T2
excited state and subsequent internal conversion
to the ground electronic state.
II. Excitation to 1A Jahn-Teller component of
1T2 state, intersystem crossing to 3A PES and
dissociation to HCH3(X).
(P.A. Cook et al., PCCP 3, 1848 (2001)).
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UV photolysis of allene and propyne.

• Two isomers of C3H4 (cyclopropene is another).
• Both are important in combustion processes, and
  are present in interstellar clouds and in the
  atmospheres of the outer planets.
• Allene contains four identical C?H bonds,
• H2CCCH2 h? ? H2CCCH H (1) D0
  30000 cm-1.
• Propyne contains two types of C?H bond, with
  different strengths
• H3CCCH h? ? H2CCCH H (2) D0
  30000 cm-1.
• H3CCCH h? ? H3CCC H (3) D0
  45000 cm-1.

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Background, I

• Both molecules can also dissociate by eliminating
  H2.
• Isomerisation on the ground (S0) state PES is
  known to occur.
• Previous photolysis at or near studies
• - Ramsay and Thistlethwaite, Can. J. Phys.
  44, 1381 (1966) UV flash photolysis of allene
  and propyne. Same transient product absorption
  detected in each case, since shown to be due to
  propargyl radical, H2CCCH.
• - Satyapal and Bersohn, JCP 95, 8004 (1991)
  CH3CCD 193 nm ? Detect D atoms only, by LIF.
• - Seki and Okabe, JCP 96, 3345 (1992)
  CD3CCH/Cl2 193 nm ? HCl only.
• - Jackson, .., ..Lee, JCP 95, 7327 (1991)
  H2CCCH2 193 nm. Angle resolved TOF-MS
  measurements of molecular products. Dominant
  primary process identified as H atom loss and
  propargyl radical formation following internal
  conversion to S0 state.

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Background, II

• Ni, .., .., and Jackson, JCP 110, 3320 (1999)
  Molecular products from 193 nm photolysis of
  allene and propyne detected by 118 nm
  photoionisation TOF-MS. Apparent differences
  in C3H3/C3H2 product ratios taken as evidence for
  direct acetylenic C?H bond fission in excited
  state of propyne.
• Sun, .., .., Neumark, JCP 110, 4363 (1999) As
  Ni et al, but used tunable VUV photoionisation at
  ALS. Apparent differences in C3H3 fragment
  photoionisation efficiency curves rationalised by
  assuming that propyne dissociates by acetylenic
  C?H bond fission.
• Chen, .., .., Rosenwaks, JCP 113, 5134 (2000)
  243.1 nm photolysis of CD3CCH(vC-H3) molecules.
  H and D atoms observed, with very similar (low)
  kinetic energy releases.
• DeSain and Taatjes, JPC A 107, 4843 (2003)
  CH3CCH 193 nm ? Monitor propargyl radical by IR
  kinetic absorption spectroscopy, time dependence
  suggests it is a primary product, quantum yield
  0.5.

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Current work
Re-investigate near UV photolysis of allene and
propyne, at several different wavelengths in the
range 193.3 - 213.3 nm, using H2CCCH2, H3CCCH and
D3CCCH precursors, and the H(D) Rydberg atom PTS
technique. Extend investigations to include
photolysis at 121.6 nm.
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Allene and propyne

• Most of the products are formed with low kinetic
  energies (i.e. the C3H3 co-fragments are formed
  with high levels of internal excitation).
• The products show no recoil anisotropy.
• A significant fraction of the products appear
  with TKERs that are only compatible with
  propargyl radical formation, i.e. channel (1) or
  (2), not (3).
• The earlier studies were very likely affected by
  secondary photolysis of the primary C3H3 and C3H2
  fragments.

TKERmax(1 or 2)
TKERmax(3)
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Propyne

• TKER spectra of H(D) atom products from propyne
  photolysis at 193.3 nm monitoring
• D atoms from D3CCCH
• H atoms from D3CCCH
• H atoms from H3CCCH
• are all very similar.
• Conclude that, in all cases, the electronically
  excited C3H4 molecules internally convert (IC) to
  high vibrational levels of the ground (S0) state,
  and then isomerise at a rate that is faster than
  their rate of unimolecular decay.

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C3H4 fragmentation channels at 193.3 nm a summary
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Allene and propyne photolysis at 121.6 nm.
H from H2CCCH2 D from D3CCCH (TOF rescaled by
2-1/2 to aid comparison) H from H3CCCH H from
D3CCCH
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direct statistical
At 121.6 nm see clear evidence for selective
acetylenic bond fission in the case of CH3CCH..
 
Parallels with results of X. Yang and coworkers,
at 157 nm. PCCP 2, 1187 (2000) JCP 112, 6656
(2000)
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• Conclusions
• Previous conclusions that 193.3 nm photolysis
  of propyne occurs mainly (or exclusively) via
  acetylenic C?H bond fission process (3) are
  wrong. Comparisons of TKER spectra obtained by
  monitoring H and D atoms from D3CCCH photolysis
  at 193.3 nm illustrate the equivalence of the
  various H(D) atoms prior to fragmentation.
• Dissociation is understandable in terms of IC
  from the initially populated S1 state,
  isomerisation on the S0 surface, and subsequent
  unimolecular decay.
• TKER spectra obtained by monitoring H atom loss
  from both allene and propyne are very similar
  again consistent with isomerisation on the C3H4
  S0 surface prior to dissociation.
• H atom TKER spectra from allene and propyne,
  and from H3CCCH and D3CCCH, are discernibly
  different when exciting at 121.6 nm (or 157 nm).
  Some fraction of the excited propyne molecules
  dissociate in a bond selective manner, by fission
  of the acetylenic C?H bond.

R.H. Qadiri et al., JCP 116, 906 (2002) JCP
(submitted).
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Acknowledgements
Colleagues Phillip Cook, Rafay Qadiri Dr Emma
Feltham, Dr Hendrik Nahler Andrew Orr-Ewing,
Colin Western, Richard Dixon, Keith
Rosser. Funding EPSRC, Leverhulme Trust, EU,
Royal Society.