Harmonium

LSU-ISIC : Thomas Barillot, Luca Longetti, Lars Mewes, Serhii Polishchuk, Michele Puppin, M. Chergui

LSE-IPhys: Alberto Crepaldi, Gianmarco Gatti, Silvan Roth, Marco Grioni

LND-ISIC: Andrew Clark, Marcel Drabbels

LUMES-IPhys: Fabrizio Carbone

An ultrafast VUV source

Christopher Arrell, José Ojeda, Luca Longetti, Serhii Polishchuk, Frank van Mourik, Majed Chergui

Photoelectron spectroscopy (PES) is an effective tool for the study of electron behaviour in matter giving understanding to their chemical and physical properties. By using a sufficiently energetic probe photon one can directly access the electronic states of valence and core levels. Time resolved PES offers even further access to electronic and structural dynamics by using pump-probe techniques.

Until recently PES has been limited to studying gas and solid samples, but the development of liquid microjets has opened up liquid phase samples to the technique. A liquid jet with a diameter of 10s μm with several millimetres of laminar flow can now be produced in high vacuum conditions. By collecting photo-emitted electrons very close to the jet, electron energies can be measured before scattering from surrounding gas vapour allowing unambiguous PE spectra to be measured.

 

Extreme ultraviolet (EUV) light can be coherently produced by high harmonic generation (HHG). This lab based process gives access to photon energies from 10s -100s eV and temporal resolution down to the attosecond regime. As part of the high harmonic beamline at the LSU an end station has been constructed to conduct time resolved PES from liquid samples.

 

The system comprises of a 1 – 15 kHz 15 W NIR source, high harmonic generation apparatus, EUV monochromator, EUV spectrometer, liquid microjet interaction chamber and a differentially pumped photoelectron spectrometer.

 

Recent results have identified and characterised the laser assisted photoelectric effect in liquids for the first time. Understanding strong-field interactions with a pump laser pulse is critical to following ultrafast charge transfers occurring on a similar timescale.

 

LAPE from liquid water, characterisation of LAPE response from cross correlation of VUV and 800 nm fields.  For more information see Arrell et al, Phys. Rev. Lett. 117 (2016) 143001 

 

Continuing recent research carried out by the group using hard X-ray absorption techniques at the Swiss light source and other in-house spctroscopies we ate currently studing the ultrafast spin flip in solvated metal complexes.

 

This activity brings together the following groups:

LSU – ISIC

LSE – IPHYS

LUMES – IPHYS

LND- ISIC

The collaboration involves 4 professors, 4 post-docs and 9 PhDs

Harmonium has been built within the framework of LACUS, in the labs of Laboratory of Ultrafast spectroscopy (LSU) of Majed Chergui in the Institute of Chemical Sciences and Engineering (ISIC). The time resolved ARPES experiment and crystal growth setup of the Laboratory of Photoelectron spectroscopy (LSE) from the Institute of Physics (IPHYS) is the responsibility of Macro Grioni.

The helium nano droplet and coincidence detection end station is the responsibility of Marcel Drabbels from the Laboratory of Molecular NanoDynamics (LND) in ISIC.

The beamline has been designed and is managed by Christopher Arrell from the LSU who is also responsible for the liquid phase end station.

The facility is open to all members of LACUS

High harmonic generation

High harmonic generation is used to produce probe light between 20 and 110 eV, using an effusive localised gas source of typically argon or neon.  A spectrum of odd harmonics is produced, with a low spatial divergence.  Argon is used to produce high intensity harmonics up to the Cooper minimum (~ 40eV), and neon gas is used for higher energy harmonics.  A typical HHG spectrum produced in neon gas is shown below.

and a spectrum produced in argon:

Monochromation

To select a signal harmonic to use as a probe, a single grating based monochromator has been built.  Using the grating in an off-plane mount the temporal broadening of the pulse is limited.  Four different gratings can be used to give either high temporal or spectral resolution of high and lower energy harmonics.  The measured specifications of the instrument are:

The measured flux on target (after focusing) of the system is 1 x 1011 photons per second for energies up to 40 eV and 5 x 108 photons per pulse for higher energies.

Laser source

Three amplifiers are used for the Harmonium source to provide 785 nm for VUV production and for frequency conversion for an optical pump.

VUV production

 

A 15 W cryogenically cooled regenerative amplifier drives the VUV source.  The output of the laser can be tuned from 3 – 15kHz providing 1 – 3 mJ of energy per pulse, while maintaining 0.3 -0.5% RMS shot-to-shot noise.  The laser centred at 785 nm, produces 45 fs FWHM pulses.

 

trARPES and Molecular beam optical pump

 

A further regenerative amplifier is electronically synchronised with the VUV production amplifier (situated 10 metres away) providing pulses at 6kHz, 45 fs pulse duration and 12 W of average power.  This seeds a OPA providing a tuneable pump from the UV to the IR.  The jitter between the VUV pulse and OPA output is circa 100 fs.

 

Liquid phase optical pump

 

A cryogenically cooled booster stage has been built to provide an optical pump for the liquid phase measurements.  The booster typically provides 6kHz, 45 fs pulse duration and 12 W of average power and seeds a TOPAS providing a tuneable pump.  The stage is directly seeded by the VUV production amplifier and is inherently synchronised.

 

Coherent 2D Setup

 

A regenerative amplifier (6kHz, 45 fs pulse duration and 12 W of average power) is also located at the Harmonium facility and is used the a coherent 2D setup.  This amplifier shares an oscillator with the VUV production amplifier.

 

The output spectrum of the laser can be spectrally broadened to over an octave using a hollow fibre pulse compressor.  The differentially pumped ~ 1 m fused silica waveguide maintains a pressure gradient of typically argon of neon.  The laser output is coupled into the fibre, and due to the high intensity of the laser self phase modulation and spectral broadening occurs.

 

The broad spectrum allows the pulse to be further compressed with chirped mirrors to ~ 8 fs. For more information click here.

End stations

The beamline allows the monochromated XUV pulse to be delivered to either of 3 future end stations.  Currently the time resolved photoemission spectroscopy liquid phase end station (Wet station) is installed and has been commissioned at the Artemis laser facility and at the Ultrafast laser group at ETHZ.  A time-resolved ARPES end station is being constructed by Prof. Grioni and Prof. Carbone, and a molecular beam line end station is under construction by Dr. Drabbels.

Summary of end station capabilities:

Wet station

TR-ArpES station

  • Cryo cooled sample preparation
  • ARPES detector
 

Molecular beam station

  • Produces a molecular beam
  • Coincidence detection