Tracking energy scale variations from scan to scan in nuclear resonant vibrational spectroscopy: In situ correction using zero-energy position drifts ΔEi rather than making in situ calibration measurements

Nuclear resonant vibrational spectroscopy (NRVS) is an excellent modern vibrational spectroscopy, in particular, for revealing site-specific information inside complicated molecules, such as enzymes. There are two different concepts about the energy calibration for a beamline or a monochromator (including a high resolution monochromator): the absolute energy calibration and the practical energy calibration. While the former pursues an as-fine-as-possible and as-repeatable-as-possible result, the latter includes the environment influenced variation from scan to scan, which often needs an in situ calibration measurement to track. However, an in situ measurement often shares a weak beam intensity and therefore has a noisy NRVS spectrum at the calibration sample location, not leading to a better energy calibration/correction in most cases. NRVS users for a long time have noticed that there are energy drifts in the vibrational spectra's zero-energy positions from scan to scan (Delta E-i), but their trend has not been explored and utilized in the past. In this publication, after providing a brief introduction to the critical issue(s) in practical NRVS energy calibrations, we have evaluated the trend and the mechanism for these zero-energy drifts (Delta E-i) and explored their link to the energy scales (alpha(i)) from scan to scan. Via detailed analyses, we have established a new stepwise procedure for carrying out practical energy calibrations, which includes the correction for the scan-dependent energy variations using Delta E-i values rather than running additional in situ calibration measurements. We also proved that one additional instrument-fixed scaling constant (a(0)) exists to convert such calibrated energy axis (E') to the real energy axis (E-real). The calibrated real energy axis (E-real) has a preliminary error bar of +/- 0.1% (the 2 sigma(E) divided by the vibrational energy position), which is 4-8 times better than that from the current practical energy calibration procedure.

Automated summary

Nuclear resonant vibrational spectroscopy (NRVS) is an excellent modern vibrational spectroscopy method for revealing site-specific information inside complex molecules such as enzymes. This study evaluates the trends and mechanisms of zero-energy drifts (Delta E-i) in NRVS spectra and explores their correlation with energy scales (alpha(i)) between scans. Through detailed analysis, a new stepwise procedure for practical energy calibration is established, which uses Delta E-i values to correct for scan-dependent energy variations instead of conducting additional in situ calibration measurements. It is also proven that an additional instrument-fixed scaling constant (a(0)) exists to convert the calibrated energy axis (E') to the real energy axis (E-real). The calibrated real energy axis (E-real) has a preliminary error bar of +/- 0.1%, which is 4-8 times better than the current practical energy calibration procedure.