Mid infrared-terahertz double resonance experiments at Jefferson Lab – a progress report. Harvey Rutt, University of Southampton Michael Klopf, Jefferson Lab The ‘JLab Team’! SR User Meeting, 14 th September 2007

What are we trying to do? THz JLab CSR source Mid IR Glowbar (initially) Sample Detect the influence of this…. On this Easy

Novelty Fully frequency multiplexed in the MIR Fully frequency multiplexed in the MIR We should see double resonance wherever it occurs in the MIR We should see double resonance wherever it occurs in the MIR CSR source has the power to do broad band excitation; PRF changeable CSR source has the power to do broad band excitation; PRF changeable pS excitation pS excitation Precise, absolute phase information Precise, absolute phase information

Reminder…. THz JLab CSR source Mid IR Glowbar (initially) Sample Detect the influence of this…. On this Easy

In reality: THz MIR

The CSR Source Major interests 74.9 MHz 37.4 MHz 18.7 MHz 9.4 MHz 4.7 MHz

Why? – gases. (Actually it wont work for CO 2 ! The molecule must have a permanent dipole moment.) THz Really a trial run for what follows. But of some interest in measuring rotational relaxation rates, and possibly for some specialist spectroscopic applications.

Why - biomolecules Amide (& other) bands are structure dependent.

Myoglobin structure – ex Wikipedia ‘THz mode’ N-H The large scale motion of the ‘THz mode’ modulates the environment of the N-H group. Can we detect this?

A few little practical issues:

You need a special MIR detector Best possible D* - as ever. Best possible D* - as ever. Crucially, fast Crucially, fast Normal 77K MCTs are typically~1MHz Normal 77K MCTs are typically~1MHz Our PRF is MHz to tens of MHz – and we would like low, stable phase shift. Our PRF is MHz to tens of MHz – and we would like low, stable phase shift. VIGO TEC cooled, ~150MHz, D* >2.5*10 9 cmHz 1/2 W -1 VIGO TEC cooled, ~150MHz, D* >2.5*10 9 cmHz 1/2 W -1 Block THz with n-Ge; BaF 2 grid polarizer to reduce residual fringes & further reduce THz. Block THz with n-Ge; BaF 2 grid polarizer to reduce residual fringes & further reduce THz.

You really need a good purge…. In the MIR, water ( cm -1 ) and CO 2 (~2340cm -1 ) are also serious problems. 1m of air

Purge problems!

Window material choices. Diamond; expensive in this aperture Diamond; expensive in this aperture High resistivity oxygen free ‘FZ’ silicon; low THz loss, some mid-IR loss but acceptable High resistivity oxygen free ‘FZ’ silicon; low THz loss, some mid-IR loss but acceptable High index leads to extremely strong FP fringes High index leads to extremely strong FP fringes Wedge angles very large Wedge angles very large Brewster angle the only solution. Brewster angle the only solution.

Doesn’t look that difficult!

Trickys gas cell:

Conical bore to reduce unsupported window area.

Signal processing The signal is frequency multiplexed. The signal is frequency multiplexed. The ‘normal’ absorption spectrum is, as usual in the low kHz region. The ‘normal’ absorption spectrum is, as usual in the low kHz region. The ‘interaction’ spectrum occurs as side- bands on an RF carrier at the FEL PRF The ‘interaction’ spectrum occurs as side- bands on an RF carrier at the FEL PRF In-phase (absolute!) is the prompt signal In-phase (absolute!) is the prompt signal In quadrature is a ‘thermal’ signal In quadrature is a ‘thermal’ signal

How? MIR Detector FTIR Spectrometer Nicolet RF Phase sensitive detector SR844 kHz region data MHz region data ‘Normal’ FTIR spectrum Phase reference from accelerator In PhaseIn quadrature Anti-Aliasing Filter Anti-Aliasing Filter Data acquisition ‘Interaction’ spectrum MIR Beam Polarizer THz block

Trivial – ground loops……. MIR Detector FTIR Spectrometer Nicolet RF Phase sensitive detector SR844 kHz region data MHz region data ‘Normal’ FTIR spectrum Phase reference from accelerator In PhaseIn quadrature MIR Beam Anti-Aliasing Filter Anti-Aliasing Filter Data acquisition All cables RG58 to RG223 & new BNC Isolation amp RF transformer HP Filter Ferrite Low jitter fibre optic 60dB better; maybe 15dB to go……………. MIR Beam Polarizer THz block ‘Interaction’ spectrum

Future work. Make it work! Make it work! Gases Gases Small proteins etc Small proteins etc Time resolved version using the MIR FEL Time resolved version using the MIR FEL Narrow band filter the THz Narrow band filter the THz Or use a THz FEL Or use a THz FEL

Acknowledgements. The entire JLab team! The entire JLab team! Kevin Jordan for extraordinary efforts in improvising purge boxes! Kevin Jordan for extraordinary efforts in improvising purge boxes! Tom Powers and Pavel Evtushenko for instrumentation help Tom Powers and Pavel Evtushenko for instrumentation help Gwyn Williams & George Neil for making it possible Gwyn Williams & George Neil for making it possible Ken Frampton in the ECS workshop for making very awkward shaped bits of stainless steel. Ken Frampton in the ECS workshop for making very awkward shaped bits of stainless steel.