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