Mid infrared-terahertz double resonance experiments at Jefferson Lab – a progress report.
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Mid infrared-terahertz double Mid infrared-terahertz double resonance experiments at resonance experiments at Jefferson Lab – a progress Jefferson Lab – a progress
report. report. Harvey Rutt, University of Southampton
Michael Klopf, Jefferson Lab
The ‘JLab Team’!
SR User Meeting, 14th September 2007
What are we trying to do?What are we trying to do?THz
JLab CSR source
Mid IRGlowbar(initially)
Sample
Detect the influence of
this….
On this
Easy
NoveltyNovelty
Fully frequency multiplexed in the MIRFully frequency multiplexed in the MIR We should see double resonance We should see double resonance
wherever it occurs in the MIRwherever it occurs in the MIR CSR source has the power to do broad CSR source has the power to do broad
band excitation; PRF changeableband excitation; PRF changeable pS excitationpS excitation Precise, absolute phase informationPrecise, absolute phase information
Reminder….Reminder….THz
JLab CSR source
Mid IRGlowbar(initially)
Sample
Detect the influence of
this….
On this
Easy
In reality:In reality:
THzMIR
1 10 100 10000
1
2
3
4
5
6
7
8
9
10
Frequency (THz)
100 MHz 100 pC 150 x 150 mr
0.1 ps 0.3 ps 1.0 ps
0.1 1 10
Wa
tts/cm
-1
Frequency (cm-1)
840 W
540 W
54 W
The CSR SourceThe CSR Source
MHzN
F1497
Major interests74.9 MHz37.4 MHz18.7 MHz9.4 MHz4.7 MHz
Why? – gases.Why? – gases.
(Actually it wont work for CO2!The molecule must have a permanent
dipole moment.)
THz THz
Really a trial run for what follows.
But of some interest in measuring rotational relaxation rates, and possibly
for some specialist spectroscopicapplications.
Why - biomoleculesWhy - biomolecules
Amide (& other)bands are
structure dependent.
Myoglobin structure – ex WikipediaMyoglobin structure – ex Wikipedia
‘THz mode’
N-H The large scale motion of the‘THz mode’ modulates the
environment of the N-Hgroup.
Can we detect this?
A few little practical issues:A few little practical issues:
You need a special MIR You need a special MIR detectordetector
Best possible D* - as ever.Best possible D* - as ever. Crucially, Crucially, fastfast Normal 77K MCTs are typically~1MHzNormal 77K MCTs are typically~1MHz Our PRF is MHz to tens of MHz – and we Our PRF is MHz to tens of MHz – and we
would like low, stable phase shift.would like low, stable phase shift. VIGO TEC cooled, ~150MHz, VIGO TEC cooled, ~150MHz,
D* >2.5*10D* >2.5*1099cmHzcmHz1/21/2WW-1-1
Block THz with n-Ge; BaFBlock THz with n-Ge; BaF22 grid polarizer to grid polarizer to reduce residual fringes & further reduce reduce residual fringes & further reduce THz.THz.
You really need a good You really need a good purge….purge….
In the MIR, water(1300-1900cm-1)
and CO2
(~2340cm-1)are also serious
problems.
1m of air
Purge problems!Purge problems!
Window material choices.Window material choices.
Diamond; expensive in this apertureDiamond; expensive in this aperture High resistivity oxygen free ‘FZ’ High resistivity oxygen free ‘FZ’
silicon; low THz loss, some mid-IR silicon; low THz loss, some mid-IR loss but acceptableloss but acceptable
High index leads to extremely strong High index leads to extremely strong FP fringesFP fringes
Wedge angles very largeWedge angles very large Brewster angle the only solution.Brewster angle the only solution.
Doesn’t look that difficult!Doesn’t look that difficult!
Trickys gas cell:Trickys gas cell:
Conical bore to reduce Conical bore to reduce unsupported window area.unsupported window area.
Signal processingSignal processing
The signal is frequency multiplexed.The signal is frequency multiplexed. The ‘normal’ absorption spectrum is, as The ‘normal’ absorption spectrum is, as
usual in the low kHz region.usual in the low kHz region. The ‘interaction’ spectrum occurs as The ‘interaction’ spectrum occurs as
side-bands on an RF carrier at the FEL side-bands on an RF carrier at the FEL PRFPRF
In-phase (absolute!) is the prompt signalIn-phase (absolute!) is the prompt signal In quadrature is a ‘thermal’ signal In quadrature is a ‘thermal’ signal
How?How?
MIR Detector
FTIR SpectrometerNicolet
RF Phase sensitive detectorSR844
kHz region data
MHz region data
‘Normal’ FTIR spectrum
Phase referencefrom
accelerator
In Phase In quadratureAnti-Aliasing
Filter Anti-AliasingFilter
Data acquisition
‘Interaction’ spectrum
MIR BeamPolarize
r
THz block
Trivial – ground loops…….Trivial – ground loops…….
MIR Detector
FTIR SpectrometerNicolet
RF Phase sensitive detectorSR844
kHz region data
MHz region data
‘Normal’ FTIR spectrum
Phase referencefrom
accelerator
In Phase In quadrature
MIR Beam
Anti-AliasingFilter Anti-Aliasing
Filter
Data acquisition
All cables RG58to RG223 & new
BNC
Isolation amp
RFtransformer
HP Filter
Ferrite
Low jitterfibre optic
60dB better; maybe 15dB to go…………….
MIR BeamPolarize
r
THz block
‘Interaction’ spectrum
Future work.Future work.
Make it work!Make it work! GasesGases Small proteins etcSmall proteins etc Time resolved version using the MIR Time resolved version using the MIR
FELFEL Narrow band filter the THzNarrow band filter the THz Or use a THz FELOr use a THz FEL
Acknowledgements.Acknowledgements.
The entire JLab team!The entire JLab team! Kevin Jordan for extraordinary efforts in Kevin Jordan for extraordinary efforts in
improvising purge boxes!improvising purge boxes! Tom Powers and Pavel Evtushenko for Tom Powers and Pavel Evtushenko for
instrumentation helpinstrumentation help Gwyn Williams & George Neil for making it Gwyn Williams & George Neil for making it
possiblepossible Ken Frampton in the ECS workshop for Ken Frampton in the ECS workshop for
making very awkward shaped bits of making very awkward shaped bits of stainless steel.stainless steel.