Vibrational spectroscopies (Raman, Infrared spectroscopies) measure the transitions between energy levels.
Lecture video on vibrational frequency and reduced mass:
6.5.1. What is the relationship between frequency of a vibration and the reduced mass of a molecule?
6.5.2. What is the relationship between wavenumbers and the reduced mass of a molecule?
References / Additional Information:
Reduced Mass and simple harmonic oscillator:
Larkin, P. (2011) Infrared and Raman Spectroscopy: Principles and Spectral Interpretation. Elsevier, Amsterdam. 230 p. ISBN-13: 978-0123869845. Chapter 2. Basic Principles.
Reduced mass derivation: https://youtu.be/iUwBW4e9n6g
Quantum mechanics playlist: https://www.youtube.com/playlist?list=PLm8ZSArAXicL3jKr_0nHHs5TwfhdkMFhh