Applications of photoelectron velocity map imaging at high resolution or Photoionization dynamics of NH 3 (B 1 E  ) Katharine L. Reid (Paul Hockett, Mick Staniforth) University of Nottingham, United Kingdom

The importance of having control over the probe wavelength in pump-probe experiments In our study of the photoionization dynamics of excited state ammonia, both of these features have been crucial. (i)Control over resolution (see Alistair Green poster – intramolecular dynamics in toluene) (ii) Awareness of continuum resonances (see Mick Staniforth poster – observation of shape resonances in small aromatic molecules)

In the absence of a molecular frame measurement a determination of the radial dipole matrix elements controlling an ionization process requires photoelectron angular distributions (PADs) to be measured for rotationally resolved states of the ion. This is a challenge for photoelectron spectroscopy! The highest resolution method of ion spectroscopy (ZEKE) does not allow the measurement of PADs Can we achieve rotational resolution in small polyatomic ions using imaging?

Slow electron velocity map imaging* *Osterwalder et al., JCP 121, 6317 (2004)

NH 3

Approaching the molecular frame X 1 A  1 (v 2 = 0; J = 1, K = 1)  B 1 E  (v 2 = 4; J = 3, K = 2)

REMPI spectrum X 1 A  1 (v 2 = 0)  B 1 E  (v 2 = 4)

One-colour vs two-colour photoelectron images X 1 A  1 (v 2 = 0; J = 1, K = 1)  B 1 E  (v 2 = 4; J = 3, K = 2)

One-colour photoelectron spectrum

Choosing the probe wavelength

A closer look...

The 1 1 ion rotational state vs probe energy In what follows, a probe wavelength of nm (E probe = cm -1 ) was chosen.

Rotationally resolved photoelectron images X 1 A  1 (v 2 = 0)  B 1 E  (v 2 = 4)

Extracted photoelectron spectra

Images for different B state (v 2 = 4) rotational levels

X 1 A  1 (v 2 = 0; J = 1, K = 1)  B 1 E  (v 2 = 4; J = 3, K = 2)

All extracted photoelectron angular distributions for v 2 = 4 solid line is a fit to

Expectations The B state is predominantly p  with some d  character. The proportions are not known. This leads to the expectation that the photoionization dynamics will be dominated by d and s waves, with a small contribution from p and f (atomic picture). Previous work (our group, Softley and coworkers) has made assumptions about the dynamics and concluded there is no contribution from higher l partial waves.

For the  LM terms |l - l|  L  l + l Any L = 6 contribution to a PAD must come from l ≥ 3 (f wave or higher). However, in order to make ion states with K = 1 or 5 selection rules require that l must be even. Several strong ion peaks assigned to K = 1 and associated with PADs having |   0 | >> 0 are observed. Therefore the ionization dynamics must involve g waves. This information is missing in a ZEKE experiment … By inspection:

There is sufficient data to fit the  LM parameters to a model to determine the radial dipole matrix elements where Based on Dixit and McKoy, 1985

Results of the fit: PADs 1 1   2 2

Results of the fit: prediction of photoelectron spectra

spdfg  spdfg  spdfg Results of the fit: parameters Phases / degrees Hockett et al. PRL, 102, (2009) Squared radial dipole matrix elements / %

Simulating PADs following excitation of v 2 = 3 using the parameters deduced from v 2 = 4

We can also predict what we would expect to see if the excitation and ionization beams were perpendicularly polarized X 1 A  1 (v 2 = 0; J = 1, K = 1)  B 1 E  (v 2 = 4; J = 3, K = 2)

Perpendicular polarizations: PADs at different detection angles predictedmeasured

Tomographic reconstruction of angular distributions taken with perpendicular polarization geometries (Thanks to Thomas Baumert!) 3 1 ion state Raw reconstruction Fit to spherical harmonics

Dependence of PADs in the perpendicular polarization geometry on ion rotational state Reconstructed and fitted Predicted

Conclusions 1.It is possible to achieve rotational resolution of NH 3 + using VMI 2.We have measured the first rotationally resolved PADs for a polyatomic molecule 3.Control over probe wavelength is essential to optimize resolution and to avoid (or study) resonances 4.Our fit to radial dipole matrix elements appears to be robust (changing vibrational level and polarization geometry) 5.PADs reveal dynamics that are missed by ZEKE 6.We have approached the molecular frame through use of alignment by excitation. However, we are missing the even- odd l phase differences which can only be obtained when parity is broken

Prospects This is probably the end of the line for rotational resolution in polyatomic molecules. However, we can make use of: (i)Rotational coherence spectroscopy (ii)Strong field alignment In both cases the ability to treat different polarization geometries will be essential.

Acknowledgements PAUL HOCKETT Mick Staniforth Dave Townsend EPSRC