diff --git a/js/photon-absorption/model/Molecule.js b/js/photon-absorption/model/Molecule.js
index 1971665f..b58b7caa 100644
--- a/js/photon-absorption/model/Molecule.js
+++ b/js/photon-absorption/model/Molecule.js
@@ -17,6 +17,7 @@ import Vector2Property from '../../../../dot/js/Vector2Property.js';
 import inherit from '../../../../phet-core/js/inherit.js';
 import merge from '../../../../phet-core/js/merge.js';
 import Tandem from '../../../../tandem/js/Tandem.js';
+import IOType from '../../../../tandem/js/types/IOType.js';
 import moleculesAndLight from '../../moleculesAndLight.js';
 import Atom from './atoms/Atom.js';
 import AtomicBond from './atoms/AtomicBond.js';
@@ -606,4 +607,10 @@ inherit( Object, Molecule, {
 // @public {number} - distance from the molecule to query a photon for absorption, in picometers
 Molecule.PHOTON_ABSORPTION_DISTANCE = PHOTON_ABSORPTION_DISTANCE;
 
+Molecule.MoleculeIO = new IOType( 'MoleculeIO', {
+  valueType: Molecule,
+  toStateObject: molecule => molecule.toStateObject(),
+  fromStateObject: Molecule.fromStateObject
+} );
+
 export default Molecule;
diff --git a/js/photon-absorption/model/MoleculeIO.js b/js/photon-absorption/model/MoleculeIO.js
deleted file mode 100644
index a485036f..00000000
--- a/js/photon-absorption/model/MoleculeIO.js
+++ /dev/null
@@ -1,21 +0,0 @@
-// Copyright 2017-2020, University of Colorado Boulder
-
-/**
- * IO Type for Molecule
- *
- * @author Sam Reid (PhET Interactive Simulations)
- * @author Andrew Adare (PhET Interactive Simulations)
- */
-
-import IOType from '../../../../tandem/js/types/IOType.js';
-import moleculesAndLight from '../../moleculesAndLight.js';
-import Molecule from './Molecule.js';
-
-const MoleculeIO = new IOType( 'MoleculeIO', {
-  valueType: Molecule,
-  toStateObject: molecule => molecule.toStateObject(),
-  fromStateObject: Molecule.fromStateObject
-} );
-
-moleculesAndLight.register( 'MoleculeIO', MoleculeIO );
-export default MoleculeIO;
\ No newline at end of file
diff --git a/js/photon-absorption/model/PhotonAbsorptionModel.js b/js/photon-absorption/model/PhotonAbsorptionModel.js
index 9c289153..0aa27373 100644
--- a/js/photon-absorption/model/PhotonAbsorptionModel.js
+++ b/js/photon-absorption/model/PhotonAbsorptionModel.js
@@ -27,9 +27,10 @@ import EnumerationIO from '../../../../phet-core/js/EnumerationIO.js';
 import inherit from '../../../../phet-core/js/inherit.js';
 import TimeSpeed from '../../../../scenery-phet/js/TimeSpeed.js';
 import PhetioObject from '../../../../tandem/js/PhetioObject.js';
+import IOType from '../../../../tandem/js/types/IOType.js';
 import NumberIO from '../../../../tandem/js/types/NumberIO.js';
 import moleculesAndLight from '../../moleculesAndLight.js';
-import MoleculeIO from './MoleculeIO.js';
+import Molecule from './Molecule.js';
 import CH4 from './molecules/CH4.js';
 import CO from './molecules/CO.js';
 import CO2 from './molecules/CO2.js';
@@ -39,7 +40,6 @@ import NO2 from './molecules/NO2.js';
 import O2 from './molecules/O2.js';
 import O3 from './molecules/O3.js';
 import Photon from './Photon.js';
-import PhotonAbsorptionModelIO from './PhotonAbsorptionModelIO.js';
 import PhotonIO from './PhotonIO.js';
 import PhotonTarget from './PhotonTarget.js';
 import WavelengthConstants from './WavelengthConstants.js';
@@ -133,7 +133,7 @@ function PhotonAbsorptionModel( initialPhotonTarget, tandem ) {
 
   this.activeMolecules = new ObservableArray( {
     tandem: tandem.createTandem( 'molecules' ),
-    phetioType: ObservableArray.ObservableArrayIO( MoleculeIO )
+    phetioType: ObservableArray.ObservableArrayIO( Molecule.MoleculeIO )
   } ); // Elements are of type Molecule.
 
   // @public (read-only) {Emitter} - emitter for when a photon is emitted from the emission point - useful in addition
@@ -161,7 +161,7 @@ function PhotonAbsorptionModel( initialPhotonTarget, tandem ) {
 
   PhetioObject.call( this, {
     tandem: tandem,
-    phetioType: PhotonAbsorptionModelIO,
+    phetioType: PhotonAbsorptionModel.PhotonAbsorptionModelIO,
     phetioState: false
   } );
 }
@@ -494,4 +494,36 @@ inherit( PhetioObject, PhotonAbsorptionModel, {
 // @public {number} - horizontal velocity of photons when they leave the emitter, in picometers/second
 PhotonAbsorptionModel.PHOTON_VELOCITY = PHOTON_VELOCITY;
 
+PhotonAbsorptionModel.PhotonAbsorptionModelIO = new IOType( 'PhotonAbsorptionModelIO', {
+  valueType: PhotonAbsorptionModel,
+
+  /**
+   * @public
+   * @param photonAbsorptionModel
+   * TODO: eliminate this legacy pattern, see https://github.com/phetsims/tandem/issues/87
+   */
+  clearChildInstances( photonAbsorptionModel ) {
+    photonAbsorptionModel.clearPhotons();
+    // instance.chargedParticles.clear();
+    // instance.electricFieldSensors.clear();
+  },
+
+  /**
+   * Create a dynamic particle as specified by the phetioID and state.
+   * @public
+   * @param {Object} photonAbsorptionModel
+   * @param {Tandem} tandem
+   * @param {Object} stateObject
+   * @returns {ChargedParticle}
+   */
+  addChildElementDeprecated( photonAbsorptionModel, tandem, stateObject ) {
+    const value = PhotonIO.fromStateObject( stateObject );
+
+    const photon = new phet.moleculesAndLight.Photon( value.wavelength, tandem );
+    photon.setVelocity( stateObject.vx, stateObject.vy );
+    photonAbsorptionModel.photons.add( photon );
+    return photon;
+  }
+} );
+
 export default PhotonAbsorptionModel;
diff --git a/js/photon-absorption/model/PhotonAbsorptionModelIO.js b/js/photon-absorption/model/PhotonAbsorptionModelIO.js
deleted file mode 100644
index ab9f5bdf..00000000
--- a/js/photon-absorption/model/PhotonAbsorptionModelIO.js
+++ /dev/null
@@ -1,46 +0,0 @@
-// Copyright 2017-2020, University of Colorado Boulder
-
-/**
- * IO Type for PhotonAbsorptionModel
- *
- * @author Sam Reid (PhET Interactive Simulations)
- */
-
-import IOType from '../../../../tandem/js/types/IOType.js';
-import moleculesAndLight from '../../moleculesAndLight.js';
-import PhotonIO from './PhotonIO.js';
-
-const PhotonAbsorptionModelIO = new IOType( 'PhotonAbsorptionModelIO', {
-  isValidValue: v => v instanceof phet.moleculesAndLight.PhotonAbsorptionModel,
-
-  /**
-   * @public
-   * @param photonAbsorptionModel
-   * TODO: eliminate this legacy pattern, see https://github.com/phetsims/tandem/issues/87
-   */
-  clearChildInstances( photonAbsorptionModel ) {
-    photonAbsorptionModel.clearPhotons();
-    // instance.chargedParticles.clear();
-    // instance.electricFieldSensors.clear();
-  },
-
-  /**
-   * Create a dynamic particle as specified by the phetioID and state.
-   * @public
-   * @param {Object} photonAbsorptionModel
-   * @param {Tandem} tandem
-   * @param {Object} stateObject
-   * @returns {ChargedParticle}
-   */
-  addChildElementDeprecated( photonAbsorptionModel, tandem, stateObject ) {
-    const value = PhotonIO.fromStateObject( stateObject );
-
-    const photon = new phet.moleculesAndLight.Photon( value.wavelength, tandem );
-    photon.setVelocity( stateObject.vx, stateObject.vy );
-    photonAbsorptionModel.photons.add( photon );
-    return photon;
-  }
-} );
-
-moleculesAndLight.register( 'PhotonAbsorptionModelIO', PhotonAbsorptionModelIO );
-export default PhotonAbsorptionModelIO;
\ No newline at end of file