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Scientists identify excitations in one molecule indirectly: they put the molecule in a controlled environment, supply energy with photons or tunneling electrons, and measure the energy-dependent response. Optical absorption, fluorescence and excitation spectra expose electronic and vibronic transitions; Raman and infrared signals reveal molecular vibrations; and scanning-tunneling methods map local electronic states or detect vibrational energy transferred by electrons. By comparing the measured energies and signal patterns with molecular models—and by testing assignments with isotope substitution or other controls—researchers determine which excitation produced each feature.
What “excitation” means in a molecule
An excitation is a change from a molecule’s lowest-energy configuration to a higher-energy state. The word can describe several physically different changes, so identifying an excitation starts by defining what moved or changed.
Electronic excitation
An electronic excitation promotes the molecule’s electrons into a different electronic state. A photon can supply the required energy, and the molecule may later release that energy as fluorescence or nonradiative heat. Optical spectra show the photon energies at which these transitions occur. Scientists infer the electronic states from those energies and from how strongly, quickly and directionally the molecule absorbs or emits light.
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Vibrational excitation
A vibrational excitation changes the quantized motion of the nuclei while the molecule remains in the same electronic state. Stretching, bending and twisting of chemical bonds each have characteristic energies. Raman, infrared and inelastic electron-tunneling measurements detect these vibrational fingerprints.
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Vibronic excitation
A vibronic transition changes both electronic and vibrational states. Its spectrum can contain a series of closely spaced lines or bands: the main electronic change accompanied by different vibrational levels. The spacing and relative intensities help assign the vibrational motion coupled to the electronic transition. A spectrum is therefore evidence about energy levels, not a direct photograph of an abstract level diagram.
How optical measurements identify electronic states
Single-molecule optical experiments first locate or isolate one molecule, then record how it interacts with light. The detector measures photons or a change in optical intensity as the excitation energy is varied.
Absorption spectroscopy
In absorption, the experiment scans the incident photon energy and looks for energies removed from the beam. A peak indicates that the molecule can absorb that energy. Peak positions identify candidate electronic transitions; additional fine structure indicates vibrational levels involved in the transition. The exact pattern depends on the molecule, temperature, surrounding material and orientation.
Fluorescence emission
After excitation, a molecule can emit a photon as it returns to a lower state. Measuring emission wavelength and intensity reveals the energies of radiative transitions and how the molecule relaxes. Emission may occur at lower energy than the absorbed photon because the molecule can lose energy through vibrations before emitting. Time-resolved detection adds the fluorescence lifetime and can distinguish relaxation pathways that overlap in a static spectrum.
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Fluorescence-excitation spectroscopy
Instead of scanning the emitted light, scientists scan the excitation laser and monitor fluorescence from the individual molecule. Peaks occur when the laser drives an allowed transition, even if the emitted photons cover a broad range. This method is useful for resolving transitions hidden by a broad emission band.
What microscopy contributes
Optical microscopy establishes that the signal comes from one localized molecule and can follow changes as that molecule blinks, rotates or switches between states. Frequency- and time-resolved measurements add information about dynamics. Polarization-dependent intensity can reveal the orientation of a transition dipole, helping distinguish transitions with similar energies but different molecular directions. The review “Single-Molecule Optics” by Florian Kulzer and Michel Orrit (Annual Review of Physical Chemistry, 2004) describes these complementary measurements.
How scientists measure vibrations in one molecule
Vibrational methods probe bond motion rather than relying solely on electronic energy. The measured signal is usually much weaker than ordinary optical emission, so enhancement, careful background subtraction or a surface-based geometry may be required.
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In Raman spectroscopy, incident light scatters with a slightly different energy. The energy difference between incoming and scattered photons equals a molecular vibrational energy. Each bond motion produces a characteristic shift, allowing scientists to assign stretches, bends and collective modes. Resonance or near-field enhancement can increase the signal sufficiently for single-molecule measurements, but the enhancement depends on the molecule and experimental configuration.
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Infrared absorption
Mid-infrared light is absorbed when its frequency matches a vibration that changes the molecule’s dipole moment. The resulting absorption bands provide a vibrational fingerprint and information about structure, bonding and energetic changes after excitation. Infrared sensitivity to an individual molecule depends strongly on how the molecule is positioned and how the optical field is concentrated. “Vibrational spectroscopy as a probe of molecule-based devices” by Anastasia B. S. Elliott, Raphael Horvath and Keith C. Gordon (Chemical Society Reviews, 2012) reviews Raman and infrared approaches for molecular systems.
Inelastic electron tunneling spectroscopy
For a molecule adsorbed on a conducting surface, a scanning tunneling microscope can inject electrons through the molecule or nearby vacuum gap. Most electrons tunnel elastically, retaining their energy. When the applied bias provides at least the energy of a molecular vibration, some electrons tunnel inelastically and lose that energy to the molecule. The extra tunneling channel appears as a feature in the conductance-versus-voltage trace, often analyzed through the second derivative of current with respect to voltage. Matching the feature energy to calculated or known vibrational modes identifies the excitation.
How STM and STS reveal local electronic structure
A scanning tunneling microscope (STM) maintains a sharp conductive tip above a conductive surface and records the tunneling current while scanning. The resulting image maps the surface with atomic-scale spatial resolution, but an image alone does not identify every molecular excitation.
Scanning tunneling spectroscopy
In scanning tunneling spectroscopy (STS), the tip is held at a selected location while the voltage is swept and the tunneling current is measured. Peaks, steps or changes in conductance indicate energies at which electronic states are available locally. STS can therefore map the spatial distribution of a molecular orbital or other local electronic feature by repeating the voltage measurement at different positions.
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Combining STS with STM-IETS
STS addresses local electronic structure, whereas STM-IETS addresses vibrational energy transfer. Running both on the same adsorbed molecule can show whether an electronic feature and a vibrational feature belong to the same site. The technique requires a suitable conducting substrate, a stable tip and a molecule that remains intact under the tunneling conditions. Harold J. W. Zandvliet and Arie van Houselt discuss STS methods in “Scanning Tunneling Spectroscopy” (Annual Review of Analytical Chemistry, 2009).
How an experiment separates different excitations
Scientists generally follow a sequence that links an observed signal to a physical transition rather than assigning a peak from its position alone.
- Prepare and isolate the molecule. The molecule may be observed in an optical environment, embedded in a host, or deposited on a conducting surface for STM measurements. The preparation determines which probes are possible.
- Choose the energy probe. Use photons when electronic or vibronic transitions are the target; use Raman or infrared light for bond vibrations; use tunneling electrons for local electronic states or surface-bound vibrational modes.
- Record an energy-resolved signal. Scan photon energy, emitted wavelength or bias voltage and measure intensity, current or conductance. For single molecules, confirm that the signal is localized and stable enough to analyze.
- Extract peak positions and patterns. Note energies, linewidths, relative intensities, polarization dependence, time constants and spatial distribution. A group of evenly spaced lines may indicate vibrational progression, while a localized conductance feature may indicate a molecular orbital or inelastic channel.
- Compare with molecular calculations and reference spectra. Quantum-chemical models predict electronic transitions and normal-mode energies. Agreement in several features is stronger evidence than matching one peak.
- Change one variable as a control. Alter isotope mass, excitation wavelength, tip position, polarization, temperature or substrate coupling. A genuine assignment should change in the way expected for that perturbation.
- Check for alternative origins. Instrumental background, substrate states, multiple molecules, charging events and tip changes can imitate molecular features. Repeating the measurement on isolated molecules and under controlled conditions helps rule them out.
How assignments are validated
Peak assignment is an inference. Scientists strengthen it by making a prediction that differs for competing explanations and then testing that prediction.
Isotope substitution
Replacing hydrogen with the heavier isotope deuterium changes the mass involved in C–H vibrations and shifts their vibrational energies. If a spectral feature moves by the predicted amount after deuteration, that supports its assignment to a C–H mode. Features associated with atoms not involved in the replaced bond should move much less. This strategy is described in the 2014 Chemical Society Reviews article “Single-molecule electronics: from chemical design to functional devices.”
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Resonance-selective enhancement
A 2024 arXiv preprint titled “Selective Excitation of Vibrations in a Single Molecule” reports tuning a laser to a vibronic resonance in deprotonated phthalocyanine molecules and observing enhanced Raman response from selected vibrations with an STM platform. The result illustrates how a resonance can make particular modes more visible; it is a reported preprint demonstration, not evidence that every single-molecule setup can routinely select any desired vibration.
Line shape, linewidth and dynamics
A narrow, stable line suggests a relatively well-defined transition, while broadening can indicate short lifetimes, environmental disorder or strong coupling to a substrate. Time-resolved optical data can connect linewidths and lifetimes to relaxation processes. In tunneling measurements, the detailed line shape and its dependence on tip position provide additional selection information.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What each method can and cannot tell you
| Method | Measured signal | Best evidence | Important limitations |
|---|---|---|---|
| Single-molecule optical spectroscopy | Absorption, emission or fluorescence intensity versus photon energy; often with time and polarization information | Electronic transitions, vibronic structure and optical dynamics | Visibility depends on transition strength, molecular orientation and environment; it does not directly image an energy level |
| Raman spectroscopy | Energy shifts of inelastically scattered photons | Vibrational modes and structural changes | Single-molecule signals may require enhancement; substrate and optical geometry affect intensity |
| Infrared spectroscopy | Absorption of mid-infrared photons | Dipole-active vibrational modes and molecular bonding | Single-molecule sensitivity depends strongly on field concentration and sample arrangement |
| STM imaging | Tunneling current while scanning position | Atomic-scale shape and location of a surface-bound molecule | An image alone does not identify all electronic or vibrational excitations |
| STS | Tunneling current or conductance versus voltage at selected positions | Local electronic states and their spatial distribution | Requires a conductive substrate and stable tip; substrate states can overlap molecular states |
| STM-IETS | Features caused by electrons losing energy to molecular vibrations | Vibrational energies of an adsorbed single molecule | Substrate coupling and selection rules can broaden, suppress or alter observable modes |
Why the molecule’s environment changes the answer
An isolated molecule does not behave exactly like the same molecule on a metal surface or inside a host material. Nearby charges can shift electronic transition energies; chemical bonding to a substrate can broaden levels; and energy can flow into the surface instead of producing detectable light. In STM-IETS, strong substrate coupling can suppress a vibrational signal or make it too broad to resolve. Therefore, a missing peak is not proof that the corresponding mode does not exist.
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Selection rules also matter. A transition may be energetically allowed but weak in a particular geometry, polarization or tunneling position. Raman and infrared methods emphasize different symmetry properties, and tunneling electrons couple most strongly to modes that modulate the tunneling pathway. Comparing methods can reveal a mode that one technique misses, but the spectra must be interpreted in their respective environments rather than treated as interchangeable fingerprints.
How can you tell which energy state a molecule is in?
You cannot usually determine a molecule’s complete state from one signal. Instead, scientists identify a set of transitions and compare their energies, intensities, lifetimes and response to controlled changes. Optical excitation and emission indicate which electronic or vibronic transition is being driven. Vibrational spectra show the nuclear motion associated with that state. STS can reveal where local electronic states are located, while time-resolved measurements show how quickly the molecule leaves an excited state.
The strongest conclusion comes when independent observations agree: for example, an optical transition has the predicted energy and polarization, a vibrational progression has the expected spacing, and isotope substitution shifts only the modes involving the replaced atoms. Even then, the assignment is specific to the molecule’s preparation and environment.
Common interpretation mistakes
- Calling every peak an electronic state. Vibrational progressions and vibronic transitions can produce multiple nearby peaks.
- Treating an STM image as a spectrum. Spatial contrast identifies shape or location; STS and IETS provide energy information.
- Assuming a missing line means a missing mode. Coupling, selection rules, noise and limited resolution can hide a real excitation.
- Ignoring the substrate. A surface can shift, broaden or quench molecular states and can contribute its own electronic features.
- Assigning a mode from one energy alone. Calculations, isotope shifts, polarization, spatial dependence or another independent measurement are needed for a robust identification.
- Generalizing one demonstration. Resonance-selective single-molecule Raman enhancement has been reported in a 2024 preprint, but performance depends on the molecule and instrument.
The practical answer
Scientists identify different excitations by matching the probe to the transition: photons for electronic and vibronic energies, Raman or infrared light for bond vibrations, and tunneling electrons for local electronic states or surface-bound vibrational energy loss. They then validate each assignment with spectral structure, spatial and time dependence, molecular calculations and controls such as isotope substitution. No single technique reveals every excitation in every molecule; the credible answer comes from combining complementary measurements while accounting for the molecule’s surroundings.
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