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Annual CAVIAR Science Meeting 14 - 15 Dec 2009 Annual CAVIAR Science Meeting 14 - 15 Dec 2009 Update on Leicester Lab Studies Mark Watkins, Alex Shillings.

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Presentation on theme: "Annual CAVIAR Science Meeting 14 - 15 Dec 2009 Annual CAVIAR Science Meeting 14 - 15 Dec 2009 Update on Leicester Lab Studies Mark Watkins, Alex Shillings."— Presentation transcript:

1 Annual CAVIAR Science Meeting Dec 2009 Annual CAVIAR Science Meeting Dec 2009 Update on Leicester Lab Studies Mark Watkins, Alex Shillings & Stephen Ball

2 Introduction Activities since summer 09 – Valve Design – Collaboration - laser apparatus loan from Cambridge – Experiments – Ab initio and training (preliminary work) Results of Activities – Valve characterisation – Pulsed detection method – Search for water monomer and dimer spectra – Valve testing with O 2 and NO 2 Problems – Issues with slit valve – Number density and detection issues Activities since summer 09 – Valve Design – Collaboration - laser apparatus loan from Cambridge – Experiments – Ab initio and training (preliminary work) Results of Activities – Valve characterisation – Pulsed detection method – Search for water monomer and dimer spectra – Valve testing with O 2 and NO 2 Problems – Issues with slit valve – Number density and detection issues

3 Rationale Monomer Red = HITRAN 04 (0.24nm FWHM) Dimer (20 cm-1 FWHM), [dimer] = 1% of [H2O] Blue = Schofield & Kjaergaard 2003 Green = Schofield et al 2007 Predicted (H 2 O) 2 overtones = 3 at 960 nm = 4 at 755 nm = 5 at 622 nm Why? Easiest to detect experimentally (red-shifted from monomer); Atmospheric importance How? Broadband cavity-based spectroscopy => v long absorption path lengths (Strong H 2 O monomer bands in field work spectra at 650nm, 570nm, 445nm) Pulsed nozzle methods => non-equilibrium dimer concentration => rotation & vibration cooling Monomer Red = HITRAN 04 (0.24nm FWHM) Dimer (20 cm-1 FWHM), [dimer] = 1% of [H2O] Blue = Schofield & Kjaergaard 2003 Green = Schofield et al 2007 Predicted (H 2 O) 2 overtones = 3 at 960 nm = 4 at 755 nm = 5 at 622 nm Why? Easiest to detect experimentally (red-shifted from monomer); Atmospheric importance How? Broadband cavity-based spectroscopy => v long absorption path lengths (Strong H 2 O monomer bands in field work spectra at 650nm, 570nm, 445nm) Pulsed nozzle methods => non-equilibrium dimer concentration => rotation & vibration cooling

4 What we were doing 6 months ago: LED-BBCEAS around 620 nm ( = 5) Beautiful spectra of H 2 O (1% in air), min absorption coeff < 10 9 cm 1 But no H 2 O (monomer) absorption detectable through pulsed valve. IssueSolution = 5 is weakMove to stronger = 4 overtone 750nm 2.Point nozzleSlit nozzle 3.[H 2 O] in source gasHeated source & nozzle 4.CW detection with LEDPulsed detection with broadband laser What we were doing 6 months ago: LED-BBCEAS around 620 nm ( = 5) Beautiful spectra of H 2 O (1% in air), min absorption coeff < 10 9 cm 1 But no H 2 O (monomer) absorption detectable through pulsed valve. IssueSolution = 5 is weakMove to stronger = 4 overtone 750nm 2.Point nozzleSlit nozzle 3.[H 2 O] in source gasHeated source & nozzle 4.CW detection with LEDPulsed detection with broadband laser

5 Experiment Concept Clocked CCD camera & spectrograph

6 Pulsed Valve Concepts Why use a General Valve? Continuous flow valves generally difficult to deal with and can easily overload turbo and diffusion pumps Use a pulsed valve to significantly reduce load on pumps and reduce resource usage General valve is an excellent source for a free jet expansion into vacuum Can use backing pressures behind a the valve in excess of 10 bar Can optimise operation of the valve for a given backing pressure The larger the backing pressure and better optimised the valve, the better the collisional and expansion cooling when the valve opens to vacuum Better cooling conditions allow for non-equilibrium generation of cluster species Backing pressure and valve operation can be optimised to bias towards a particular cluster size; higher backing pressure and better optimisation favour larger cluster formation Why use a General Valve? Continuous flow valves generally difficult to deal with and can easily overload turbo and diffusion pumps Use a pulsed valve to significantly reduce load on pumps and reduce resource usage General valve is an excellent source for a free jet expansion into vacuum Can use backing pressures behind a the valve in excess of 10 bar Can optimise operation of the valve for a given backing pressure The larger the backing pressure and better optimised the valve, the better the collisional and expansion cooling when the valve opens to vacuum Better cooling conditions allow for non-equilibrium generation of cluster species Backing pressure and valve operation can be optimised to bias towards a particular cluster size; higher backing pressure and better optimisation favour larger cluster formation

7 Valve Concepts However, there are certain design limitations when using a valve – The number density in the jet expansion from the valve decreases rapidly within a short distance of the valve orifice. – Pulsed operation Duty cycle is very low compared to a continuous wave (LED) source Continuous integration of signal leads to very low data acquisition time with respect to dead time Upper limit to data collection time per second given by the product of the opening time of the valve (~ 250 s) and the base repetition rate of the experiment (10 – 20 Hz) – Low absorption path length The path length over which absorption takes place is much smaller than the total path length of the cavity However, there are certain design limitations when using a valve – The number density in the jet expansion from the valve decreases rapidly within a short distance of the valve orifice. – Pulsed operation Duty cycle is very low compared to a continuous wave (LED) source Continuous integration of signal leads to very low data acquisition time with respect to dead time Upper limit to data collection time per second given by the product of the opening time of the valve (~ 250 s) and the base repetition rate of the experiment (10 – 20 Hz) – Low absorption path length The path length over which absorption takes place is much smaller than the total path length of the cavity

8 Pulsed Broadband Laser Solution to pulsed valve operation: – Use intense [laser] source instead of LED; order of magnitude more intense and can be operated in a pulsed manner; use Cambridge broadband emission dye laser – Data collection can be taken selectively with pulsed operation; use the Cambridge BBCRDS system – Use of pulsed laser and data acquisition system mean that data collection process is synchronised to the operation of the General Valve – All dead time eliminated by switching to pulsed operation Solution to pulsed valve operation: – Use intense [laser] source instead of LED; order of magnitude more intense and can be operated in a pulsed manner; use Cambridge broadband emission dye laser – Data collection can be taken selectively with pulsed operation; use the Cambridge BBCRDS system – Use of pulsed laser and data acquisition system mean that data collection process is synchronised to the operation of the General Valve – All dead time eliminated by switching to pulsed operation Pulsed laser BBCRDS Pulsed nozzle LED-BBCEAS Nozzle: rep rate Hz, 180 – 300 s opening time

9 Pulsed Broadband Laser Ringdown time of cavity inside vacuum chamber (cavity length = 60 cm) Ringdown time ( s) Wavelength (nm) Path = 10 km Path = 5 km

10 Jet expansion Example of a free jet expansion: Picture taken from: J Chem Phys, 2009, 131, Barrel Shock MACH disk region 2 cm

11 Valve Concepts Number density issues – Number density decrease drastically with increasing separation from the valve orifice Number density issues – Number density decrease drastically with increasing separation from the valve orifice Orifice = 100 micron; 200 micron; 500 micron Mach number vs distance Density vs distance

12 Schematic of a free jet expansion Most useful spectroscopic region is in the zone of silence just prior to the MACH disk in the expansion; vibrationally and rotationally coolest region of the expansion, so will have the highest density of cluster species Physical issues with free jet expansion: – We MUST do our detection in the first 1 – 2 mm after the nozzle, after which the number density is predicted to fall off Most useful spectroscopic region is in the zone of silence just prior to the MACH disk in the expansion; vibrationally and rotationally coolest region of the expansion, so will have the highest density of cluster species Physical issues with free jet expansion: – We MUST do our detection in the first 1 – 2 mm after the nozzle, after which the number density is predicted to fall off

13 Valve Concepts Valve design: – Use a slit nozzle in preference to a circular nozzle to increase absorption path length Valve design: – Use a slit nozzle in preference to a circular nozzle to increase absorption path length beampath Slit width = 12 mm Diameter ~ 350 m

14 Heated Valve Must further modify valve in order to increase dimer production – Use a heated reservoir immediately behind valve – Increases water monomer vapour pressure considerably in region immediately behind valve – Allows for increased dimer production in free jet expansion Must further modify valve in order to increase dimer production – Use a heated reservoir immediately behind valve – Increases water monomer vapour pressure considerably in region immediately behind valve – Allows for increased dimer production in free jet expansion H 2 O reservoir General Valve thermal body heating collar thermocouple

15 Valve progress Heated nozzle and temperature controller characterisation – Both nozzle reservoir and temperature controller builds have been completed – Temperature controller has been trained in air and in vacuum up to ~60 o C – Achieve temperature control over nozzle and reservoir better than ~0.4 o C Valve characterisation – Tested both slit and circular orifice valves at a range of (i) backing pressures, (ii) opening times, (iii) temperatures – Attempted to optimise valves in all the conditions above to gain, at minimum, a reasonable estimate of viable operating parameters under a range of conditions – Tested in air for sharpness of gas pulse for comparison to performance in vacuum – Issues with slit nozzle when working at higher backing pressures and opening times, and at high temperature Heated nozzle and temperature controller characterisation – Both nozzle reservoir and temperature controller builds have been completed – Temperature controller has been trained in air and in vacuum up to ~60 o C – Achieve temperature control over nozzle and reservoir better than ~0.4 o C Valve characterisation – Tested both slit and circular orifice valves at a range of (i) backing pressures, (ii) opening times, (iii) temperatures – Attempted to optimise valves in all the conditions above to gain, at minimum, a reasonable estimate of viable operating parameters under a range of conditions – Tested in air for sharpness of gas pulse for comparison to performance in vacuum – Issues with slit nozzle when working at higher backing pressures and opening times, and at high temperature

16 Slit valve Issues with pulsed valve operation: – The nozzle armature has a flat plate with a rubberised disk mounted on it. The rubber disk distorts to form a seal against the slit orifice when the valve is shut. – This caused two immediate problems: Higher pressures force the flat plate above the rubber seal against the nozzle; valve becomes inoperable above about a 2 bar pressure differential Higher temperatures (above ~ 45 o C) caused the rubber seal to become tacky and stick to the slit nozzle, causing operation problems; typically, large pressure fluctuations were initially observed, eventually resulting in loss of operation as the seal became permanently stuck to the slit nozzle Solutions: – Buy or manufacture new faceplate that is less sensitive to temperature – Operate at low pressure – fortunately, this favours production of smaller clusters – Switch to circular orifice valve as a stopgap Issues with pulsed valve operation: – The nozzle armature has a flat plate with a rubberised disk mounted on it. The rubber disk distorts to form a seal against the slit orifice when the valve is shut. – This caused two immediate problems: Higher pressures force the flat plate above the rubber seal against the nozzle; valve becomes inoperable above about a 2 bar pressure differential Higher temperatures (above ~ 45 o C) caused the rubber seal to become tacky and stick to the slit nozzle, causing operation problems; typically, large pressure fluctuations were initially observed, eventually resulting in loss of operation as the seal became permanently stuck to the slit nozzle Solutions: – Buy or manufacture new faceplate that is less sensitive to temperature – Operate at low pressure – fortunately, this favours production of smaller clusters – Switch to circular orifice valve as a stopgap

17 Synchronising valve & laser Cambridge Apparatus – searching for absorbing species – Collaboration with Alex Shillings (Cambridge) Spatial overlap – 1 to 2 mm immediately below the nozzle orifice (optimum clustering &number density issues) – Left / right translation to intercept laser beam Temporal overlap – Laser pulse & gas pulse sync via delay generator – Temporal profile of the gas pulse (optimum clustering when valve first opens) – Cant use long gas pulses = overload pumps – Optimum opening time varies with backing pressure Clear that best chance of observing the water monomer & dimer is to optimise valve operation first with a strong absorber Cambridge Apparatus – searching for absorbing species – Collaboration with Alex Shillings (Cambridge) Spatial overlap – 1 to 2 mm immediately below the nozzle orifice (optimum clustering &number density issues) – Left / right translation to intercept laser beam Temporal overlap – Laser pulse & gas pulse sync via delay generator – Temporal profile of the gas pulse (optimum clustering when valve first opens) – Cant use long gas pulses = overload pumps – Optimum opening time varies with backing pressure Clear that best chance of observing the water monomer & dimer is to optimise valve operation first with a strong absorber

18 Synchronising valve & laser – In conjunction with uvizen spectrograph data, we have used the Cambridge clocked CCD camera to search for absorbances in: – Column abundance per pass: 1 Torr O 2 over full cavity length 1 Atm O 2 through slit valve – That we havent reliably seen O 2 through valve suggests something still not right with timings and/or spatial overlap. – Search for strongly absobing species to align valve is still in progress AbsorberAmbient airReduced pressure inside chamber Through pulsed valve into chamber at Torr Water monomerYes-No Oxygen moleculeYesYes, down to P tot = 5 Torr (20% in air) Maybe through slit valve (1 Atm, 100% O 2 ) NO 2 diluted in airYes-No Water dimer--No

19 Summary Optimisation of valve parameters proving very difficult due to small absorption, teardrop profile of pulsed expansion and changing number density through beam. Currently most likely sub-optimal spatial and temporal overlaps. Seed a very strong absorber in region of free jet expansion Pre-amplify laser beam during its second pass through the dye cell – will raise photon flux by up to an order of magnitude and enable greater signal averaging. Give ourselves best chance of seeing a detectable absorption: – (i) slit valve to increase axial overlap with laser beam – (ii) heated valve to increase [H 2 O] in source gas Problems with slit valve – sticking at higher temperatures and interferring with pulsed operation; will require new parts if available or re-manufacture if not Optimisation of valve parameters proving very difficult due to small absorption, teardrop profile of pulsed expansion and changing number density through beam. Currently most likely sub-optimal spatial and temporal overlaps. Seed a very strong absorber in region of free jet expansion Pre-amplify laser beam during its second pass through the dye cell – will raise photon flux by up to an order of magnitude and enable greater signal averaging. Give ourselves best chance of seeing a detectable absorption: – (i) slit valve to increase axial overlap with laser beam – (ii) heated valve to increase [H 2 O] in source gas Problems with slit valve – sticking at higher temperatures and interferring with pulsed operation; will require new parts if available or re-manufacture if not

20 Undergraduate BSc project: Sadna Don Ab initio calculations – water dimer and monomer – Initially started as a BSc project, but ran quickly and successfully enough to be of possible use in predicting properties of some value to our experiments. Currently raw data is being generated and worked up Test model chemistry against experiment and current ab initio – Geometries of dimer – Frequencies (harmonic and anharmonic) – Binding energies Interaction energies Dissociation energies Zero-point corrections Test of model chemistry – Comparison of above to experiment in the literature – Comparison of above to ab initio in the literature Predict further properties – Use literature basis set extrapolation techniques – Predict extrapolated frequency red-shifts VERY PRELIMINARY WORK – not necessarily of any more use than work already in the literature, but worth following up as raw data is available Ab initio calculations – water dimer and monomer – Initially started as a BSc project, but ran quickly and successfully enough to be of possible use in predicting properties of some value to our experiments. Currently raw data is being generated and worked up Test model chemistry against experiment and current ab initio – Geometries of dimer – Frequencies (harmonic and anharmonic) – Binding energies Interaction energies Dissociation energies Zero-point corrections Test of model chemistry – Comparison of above to experiment in the literature – Comparison of above to ab initio in the literature Predict further properties – Use literature basis set extrapolation techniques – Predict extrapolated frequency red-shifts VERY PRELIMINARY WORK – not necessarily of any more use than work already in the literature, but worth following up as raw data is available

21 Additional Possibilities Additional experiment – photoacoustic spectra – Very easy to set up – Time soon available on the IR OPO needed for these experiments – Signal against zero background Additional experiment – photoacoustic spectra – Very easy to set up – Time soon available on the IR OPO needed for these experiments – Signal against zero background Pulsed IR laser (OPO/OPA) Water vapour spectrum (taken in air) Signal integration and analysis


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