A high power pulsed laser system has been installed on the high magnetic field muon spectrometer ... more A high power pulsed laser system has been installed on the high magnetic field muon spectrometer (HiFi) at the International Science Information Service pulsed neutron and muon source, situated at the STFC Rutherford Appleton Laboratory in the UK. The upgrade enables one to perform light-pump muon-probe experiments under a high magnetic field, which opens new applications of muon spin spectroscopy. In this report we give an overview of the principle of the HiFi laser system and describe the newly developed techniques and devices that enable precisely controlled photoexcitation of samples in the muon instrument. A demonstration experiment illustrates the potential of this unique combination of the photoexcited system and avoided level crossing technique.
1School of Physics and Astronomy, Queen Mary University of London, London, UK. 2College of Physic... more 1School of Physics and Astronomy, Queen Mary University of London, London, UK. 2College of Physical Sciences and Technology, Sichuan University, Chengdu, People’s Republic of China. 3ISIS Muon Facility, Rutherford Appleton Laboratory, Didcot, UK. 4Department of Chemistry, University of Kentucky, Lexington, KY, USA. 5Department of Materials Science and Engineering, University of Sheffield, Sheffield, UK. 6Department of Physics, University of Fribourg, Fribourg, Switzerland. 7Materials and Life Science Division, J-PARC Center, Tokai, Japan. 8Nishina Centre, RIKEN-RAL, Wako, Japan. 9School of Biological and Chemical Sciences, Queen Mary University of London, London, UK. ✉e-mail: P.Heathcote@qmul.ac.uk; A.J.Drew@qmul.ac.uk We reported1 changes in the muonium (Mu) addition rate at different sites in 6,13-bis(tri(isopropyl)silylethynyl)-pentacene (TIPS-Pn) in CH2Cl2 when it is photo-excited compared with the ground electronic state. Scheuermann and McKenzie report in a Matters Arising2 that their independent analysis of the transverse-field muon spin rotation (TF-μSR) and avoided level crossing (ALC)-μSR spectra in ref. 1 indicate that there is no statistically significant effect of photo-excitation on this rate. One of the main criticisms from Scheuermann and McKenzie2 is that effective rate constants for Mu addition in other smaller molecules are in excess of 109 M−1 s−1, and that this is too large to be compatible with our interpretation of our TIPS-Pn data. For example, one would expect significant effects on amplitude when the rate of conversion of muons to muoniated radicals is between about 0.1 and 10 μs−1, which would correspond to a rate constant between about 106 and 108 M−1 s−1 in the TIPS-Pn concentration measured in our experiment (Supplementary Information). We note that Scheuermann and McKenzie2 are assuming that the rate constant for this reaction in TIPS-Pn is the same as for smaller molecules, as this constant has not been measured in TIPS-Pn. We have provided (Supplementary Information) a comprehensive dataset that indicates that the effective rate constant for these reactions is below 109 M−1 s−1, and probably between 107 and 108 M−1 s−1—within the range required for our interpretation to be correct. Scheuermann and McKenzie argue that the Fourier-transformed TF-μSR spectra presented in ref. 1 cannot establish and quantify any effect of illumination with statistical significance. They were unable to reproduce some of the analysis we presented1 using a standard methodology for the interpretation of Mu chemistry experiments. As discussed in the Supplementary Information, Scheuermann and McKenzie2 fitted two overlapping ALC resonances to a double Lorentzian; however, the fit parameters will have significant covariance and there are insufficient points to reliably fit the number of parameters in their fits, resulting in significant uncertainties on the extracted parameters. We note that the transverse-field fits by Scheuermann and McKenzie also seem to be all-parameter fits, which may be physically unrealistic. For example, the applied field in both light-on and light-off data is identical, and so the frequency, amplitude and phase of the low-frequency component will have shared parameters. We agree that some of our data are challenging to analyse, and we had considered the alternative interpretation and analysis methods presented by Scheuermann and McKenzie. As detailed in the supplementary information of ref. 1, we demonstrated a set of analyses that are consistent across multiple μSR experimental geometries and techniques, with the key results agreeing with independent results from optical spectroscopy3 following photo-excitation of TIPS-Pn. In particular, there is a match between the decay rate of a triplet detected by optical spectroscopy3 and that detected by μSR (Fig. 2c in ref. 1 and Supplementary Fig. 13a), confirming that μSR measures photo-excitation and that the light-induced changes are not an artefact of data analysis. We believe that the body of evidence provided indicates that our initial conclusions and interpretation1 are correct, but that further work is needed to develop the photoμSR technique; such efforts are in progress.
Light is used as a source of energy by eukaryotes, prokaryotes and archaea. Photosynthetic archae... more Light is used as a source of energy by eukaryotes, prokaryotes and archaea. Photosynthetic archaea such as Halobacterium harness light energy through a protein called bacteriorhodopsin, which operates as a light-driven proton pump [1]. In eukaryotes and prokaryotes, pigment–protein complexes use light energy to drive a series of electron transfer reactions that are coupled to the translocation of protons across a charge-impermeable membrane. The proton electrochemical gradient established by this process is used to drive the synthesis of ATP and other energy-dependent processes. In the photosynthetic apparatus of prokaryotes and eukaryotes, antenna proteins are responsible for harvesting light energy. These complexes consist of a
A high power pulsed laser system has been installed on the high magnetic field muon spectrometer ... more A high power pulsed laser system has been installed on the high magnetic field muon spectrometer (HiFi) at the International Science Information Service pulsed neutron and muon source, situated at the STFC Rutherford Appleton Laboratory in the UK. The upgrade enables one to perform light-pump muon-probe experiments under a high magnetic field, which opens new applications of muon spin spectroscopy. In this report we give an overview of the principle of the HiFi laser system and describe the newly developed techniques and devices that enable precisely controlled photoexcitation of samples in the muon instrument. A demonstration experiment illustrates the potential of this unique combination of the photoexcited system and avoided level crossing technique.
1School of Physics and Astronomy, Queen Mary University of London, London, UK. 2College of Physic... more 1School of Physics and Astronomy, Queen Mary University of London, London, UK. 2College of Physical Sciences and Technology, Sichuan University, Chengdu, People’s Republic of China. 3ISIS Muon Facility, Rutherford Appleton Laboratory, Didcot, UK. 4Department of Chemistry, University of Kentucky, Lexington, KY, USA. 5Department of Materials Science and Engineering, University of Sheffield, Sheffield, UK. 6Department of Physics, University of Fribourg, Fribourg, Switzerland. 7Materials and Life Science Division, J-PARC Center, Tokai, Japan. 8Nishina Centre, RIKEN-RAL, Wako, Japan. 9School of Biological and Chemical Sciences, Queen Mary University of London, London, UK. ✉e-mail: P.Heathcote@qmul.ac.uk; A.J.Drew@qmul.ac.uk We reported1 changes in the muonium (Mu) addition rate at different sites in 6,13-bis(tri(isopropyl)silylethynyl)-pentacene (TIPS-Pn) in CH2Cl2 when it is photo-excited compared with the ground electronic state. Scheuermann and McKenzie report in a Matters Arising2 that their independent analysis of the transverse-field muon spin rotation (TF-μSR) and avoided level crossing (ALC)-μSR spectra in ref. 1 indicate that there is no statistically significant effect of photo-excitation on this rate. One of the main criticisms from Scheuermann and McKenzie2 is that effective rate constants for Mu addition in other smaller molecules are in excess of 109 M−1 s−1, and that this is too large to be compatible with our interpretation of our TIPS-Pn data. For example, one would expect significant effects on amplitude when the rate of conversion of muons to muoniated radicals is between about 0.1 and 10 μs−1, which would correspond to a rate constant between about 106 and 108 M−1 s−1 in the TIPS-Pn concentration measured in our experiment (Supplementary Information). We note that Scheuermann and McKenzie2 are assuming that the rate constant for this reaction in TIPS-Pn is the same as for smaller molecules, as this constant has not been measured in TIPS-Pn. We have provided (Supplementary Information) a comprehensive dataset that indicates that the effective rate constant for these reactions is below 109 M−1 s−1, and probably between 107 and 108 M−1 s−1—within the range required for our interpretation to be correct. Scheuermann and McKenzie argue that the Fourier-transformed TF-μSR spectra presented in ref. 1 cannot establish and quantify any effect of illumination with statistical significance. They were unable to reproduce some of the analysis we presented1 using a standard methodology for the interpretation of Mu chemistry experiments. As discussed in the Supplementary Information, Scheuermann and McKenzie2 fitted two overlapping ALC resonances to a double Lorentzian; however, the fit parameters will have significant covariance and there are insufficient points to reliably fit the number of parameters in their fits, resulting in significant uncertainties on the extracted parameters. We note that the transverse-field fits by Scheuermann and McKenzie also seem to be all-parameter fits, which may be physically unrealistic. For example, the applied field in both light-on and light-off data is identical, and so the frequency, amplitude and phase of the low-frequency component will have shared parameters. We agree that some of our data are challenging to analyse, and we had considered the alternative interpretation and analysis methods presented by Scheuermann and McKenzie. As detailed in the supplementary information of ref. 1, we demonstrated a set of analyses that are consistent across multiple μSR experimental geometries and techniques, with the key results agreeing with independent results from optical spectroscopy3 following photo-excitation of TIPS-Pn. In particular, there is a match between the decay rate of a triplet detected by optical spectroscopy3 and that detected by μSR (Fig. 2c in ref. 1 and Supplementary Fig. 13a), confirming that μSR measures photo-excitation and that the light-induced changes are not an artefact of data analysis. We believe that the body of evidence provided indicates that our initial conclusions and interpretation1 are correct, but that further work is needed to develop the photoμSR technique; such efforts are in progress.
Light is used as a source of energy by eukaryotes, prokaryotes and archaea. Photosynthetic archae... more Light is used as a source of energy by eukaryotes, prokaryotes and archaea. Photosynthetic archaea such as Halobacterium harness light energy through a protein called bacteriorhodopsin, which operates as a light-driven proton pump [1]. In eukaryotes and prokaryotes, pigment–protein complexes use light energy to drive a series of electron transfer reactions that are coupled to the translocation of protons across a charge-impermeable membrane. The proton electrochemical gradient established by this process is used to drive the synthesis of ATP and other energy-dependent processes. In the photosynthetic apparatus of prokaryotes and eukaryotes, antenna proteins are responsible for harvesting light energy. These complexes consist of a
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