# Chemiluminescence in Hydro-Carbon Flames — CH* and OH*

## Overview

Chemiluminescence in flames arises when chemical reactions produce **excited radicals** that emit photons as they relax.

The two most important species for combustion diagnostics are:

- CH* (≈ 431 nm, blue)
    
- OH* (≈ 310 nm, UV)
    

These are not just “present radicals” — they are **selectively excited products of specific reactions**, and their relative intensity encodes combustion chemistry.

---

## Key Reactions

### CH* Formation (fuel breakdown zone)

CH* is associated with hydrocarbon fragmentation and early flame chemistry.

Typical pathways:

$$  
\mathrm{C_2H + O \rightarrow CH^* + CO}  
$$

$$  
\mathrm{CH_2 + O \rightarrow CH^* + OH}  
$$

$$  
\mathrm{C + H_2 \rightarrow CH^* + H}  
$$

These reactions dominate in **fuel-rich or near-front regions** of the flame.

---

### OH* Formation (oxidation zone)

OH* is associated with high-temperature oxidation chemistry.

Typical pathways:

$$  
\mathrm{H + O_2 \rightarrow OH^* + O}  
$$

$$  
\mathrm{O + H \rightarrow OH^*}  
$$

$$  
\mathrm{H_2 + O \rightarrow OH^* + H}  
$$

These reactions dominate in **lean or well-oxidised regions**.

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## Why Excited States Are Produced

A chemical reaction releases energy:

$$  
\mathrm{A + B \rightarrow C + D + energy}  
$$

This energy can be distributed into:

- Translational motion (heat)
    
- Vibrational energy
    
- Rotational energy
    
- **Electronic excitation**
    

If enough energy is deposited into the electronic degree of freedom:

$$  
\mathrm{OH^* \rightarrow OH + h\nu}  
$$

$$  
\mathrm{CH^* \rightarrow CH + h\nu}  
$$

The emitted photon is what we detect.

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## Non-Equilibrium Nature

These reactions occur under:

- High temperature
    
- Fast radical–radical collisions
    
- Non-equilibrium conditions
    

Therefore:

> Products are often formed **directly in excited states**, rather than being thermally excited.

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## Why Only Certain Species Emit Strongly

Not all reactions produce visible/UV light. Strong chemiluminescence requires:

- Sufficient reaction energy (~2–5 eV)
    
- Good overlap between reaction geometry and excited state (Franck–Condon principle)
    
- Fast radiative decay
    

CH* and OH* satisfy these conditions.

---

## Energy Scale

Typical photon energies:

$$  
E = h\nu  
$$

CH* emission (~431 nm):

$$  
E \approx 2.9 \text{ eV}  
$$

OH* emission (~310 nm):

$$  
E \approx 4.0 \text{ eV}  
$$

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## Spatial Structure of the Flame

- CH* peaks in the **reaction front** (fuel breakdown)
    
- OH* peaks slightly downstream in the **oxidation zone**
    

Thus, they probe different regions of the flame.

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## The OH*/CH* Ratio

Define:

$$  
R = \frac{OH^*}{CH^*}  
$$

This ratio reflects:

$$  
R \propto \frac{\text{oxidation rate}}{\text{fuel fragmentation rate}}  
$$

---

### Interpretation

- Fuel-rich:
    
    - High CH*
        
    - Lower OH*
        
    - $$R \text{ small}$$
        
- Lean:
    
    - High OH*
        
    - Lower CH*
        
    - $$R \text{ large}$$
        
- Stoichiometric:
    
    - Balanced
        
    - $$R \text{ intermediate}$$
        

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## Why the Ratio is Useful

Absolute intensity depends on:

- Optical alignment
    
- Flame size
    
- Window fouling
    
- Sensor gain
    

But the ratio:

$$  
\frac{OH^*}{CH^*}  
$$

cancels many of these effects and provides a **robust indicator of combustion state**.

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## Physical Interpretation

- CH* → “fuel is still breaking apart”
    
- OH* → “oxidation is occurring”
    

So:

> The ratio indicates where the flame sits between **fuel-dominated** and **oxidation-dominated** chemistry.

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## Deeper Insight

Even though both species arise from excited states:

- CH* is fed by **hydrocarbon radical pathways**
    
- OH* is fed by **oxidation pathways**
    

Thus the ratio is not just energy-related — it reflects:

> **Which chemical pathways dominate locally**

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## Summary

- Chemiluminescence arises from **excited radicals formed during reactions**
    
- CH* and OH* originate from **different parts of the combustion network**
    
- Their ratio provides a **real-time optical measure of stoichiometry and flame structure**
    

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