Spontaneous Raman Spectrometer

Do molecular lines keep the same energy shift when the laser changes? Yes. A vibrational transition takes a fixed amount of photon energy, so changing the excitation moves the outgoing wavelength while preserving the Raman shift. Recovering it requires a calibrated spectral axis and enough resolution to separate nearby lines.

Calibrate a spectrometer, acquire weak scattered light and compare two excitation wavelengths. Separate a close pair of synthetic vibrational modes from fluorescence and counting noise.

Turn scattered light into two bands

Drag the slit jaw to merge or separate the recorded CCD bands.

Excitation laser

\lambda_0

532\,\mathrm{nm}

Exposure

t/\mathrm{s}

2

Instrumental response FWHM

\Gamma_{\rm inst}/\mathrm{cm}^{-1}

8

Calibration and advanced settings

Indicated wavelength offset

\delta\lambda/\mathrm{nm}

0.2

Fluorescence count rate per bin

b/\mathrm{s}^{-1}

1

Sampling seed

s_0

51

Calibrated wavelength axis

The slit maps to instrumental response width; the molecular linewidth stays fixed. Changing the axis offset requires a new reference.

Model assumptions

This scalar teaching instrument assumes two nonresonant Stokes modes, a smooth fluorescence background and a Gaussian response. The mode positions and photon rates describe a synthetic material. They are not measured material data. The wavelength axis has an adjustable offset; a separately acquired reference at a known shift estimates that offset. Counts are seeded Poisson samples. The model omits polarization, resonance, damage, self-absorption, spatial mapping and anti-Stokes thermometry. Optical routing and magnification are schematic. Colors identify paths, including invisible infrared light.

What the CCD counted

Simulated measurements

Move a control or reset to start a fresh simulation.

Physical assumptions and sources

Two synthetic modes are broadened and integrated over physical wavelength-bin edges. A separate known reference estimates the wavelength offset. Background is estimated from edge bins, so broad backgrounds or unresolved overlap limit peak recovery. Intensities and widths are assumed; the model does not identify a real substance. Apparatus sizes and ray angles are schematic. Physical CCD columns use a fixed brightness tone map of the selected counts. Moving packets replay equal-mass quantiles of these recorded bins; their sizes and angles are display coding. Slit width changes only the prescribed instrumental response, at fixed throughput and intrinsic molecular width.

HORIBA · MacroRAM educational laboratory
NIST · spectral intensity calibration

One-sentence intuition: A fixed vibrational energy gives a fixed reciprocal-wavelength difference, not a fixed wavelength difference.

Core mathematical model

Wavelength to Raman shift

\Delta\widetilde\nu=10^7\left(\frac1{\lambda_0}-\frac1{\lambda_s}\right)\;\mathrm{cm}^{-1}

Both wavelengths are inserted in nanometres. Positive shift means the photon left energy in the sample.

Energy transferred to the vibration

\Delta E=hc(100\Delta\widetilde\nu)

The factor of one hundred converts inverse centimetres into inverse metres. The mode energy is held fixed when the excitation changes.

Independent photon counts

N_i\sim\operatorname{Poisson}(\mu_i),\qquad\operatorname{Var}N_i=\mu_i

Each detector bin records its own signal and background photons. Subtracting an estimated background does not remove their counting noise.

Common difficulties

A smooth spectrum can hide two lines

Typical misconceptionEvery visible maximum is one molecular vibration, and smoothing can always recover a hidden pair.

Better mental modelFinite response blends nearby lines. The analysis requires two separated maxima with a resolved valley; otherwise it reports the pair as unresolved.

Run the experiment

01
Calibrate before acquiring the sample
Use Resolve the pair and Acquire calibration reference. Acquire the sample, then analyze stored samples.
What to observe: The known reference estimates the wavelength offset. The sample analysis uses the stored counts and that frozen calibration.
02
Change the laser
Select Change excitation, calibrate the second laser, and acquire another sample. Switch the graph between wavelength and Raman shift.
What to observe: The wavelength peaks move farther from the infrared source, but the recovered energy shifts agree within counting and calibration errors.
03
Test resolution and background
Acquire the Blurred doublet and Strong fluorescence presets.
What to observe: Broadening can merge the pair. More background increases fluctuations even when its smooth trend is subtracted.

Spontaneous Raman Spectrometer

What the CCD counted

Recorded photons

Recover the energy shift

Read a molecular energy difference

Core mathematical model

Wavelength to Raman shift

Energy transferred to the vibration

Independent photon counts

Common difficulties

A smooth spectrum can hide two lines

Run the experiment

Calibrate before acquiring the sample

Change the laser

Test resolution and background

Related experiments and articles

Articles

Vibrations, echoes and frequency memory

Molecules, forces and ultrafast dynamics