Photochemical reactions have the following characteristics : a.)These require photons to interact with chemical species b.)Each molecule absorbs only one photon to get itself excited c.)It is not necessary for an excited molecule to show always chemical change d.)Light is essential for a photochemical reaction
Hint : Light is the basic need for a photochemical reaction. Light contains packets of energy called photons which interact with molecules resulting in excitation of the molecule. Each photon may carry the same amount of energy and in a similar way, different types of molecules require a different amount of energy for excitation.
Complete answer : For this answer, we need to look at what photochemical reactions are. Photochemical reactions as the word define itself- Photo means light and chemical mean the change in the chemistry of the molecule. These are the reactions which use energy from light to excite the molecules in higher states and then the excited molecule reacts giving products. Now, analysing the options one by one; the first option says that photochemical reactions require photons to interact with chemical species. This statement is true because photons provide energy to molecules for excitation and then they interact with chemical species. So, the first option is correct. Now, the second option is that each molecule absorbs only one photon to get itself excited. This statement can not be true because each molecule requires some specific amount of energy for excitation. This amount of energy can be different for two molecules. The photon contains energy according to the wavelength of light used. It can be possible that the amount of energy that a molecule requires can be less than or more than associated with a single photon. So, this is incorrect. Now, for the third option which says that it is not necessary for an excited molecule to always show chemical change. Yes, this statement can be true because when a molecule gets excited; it can undergo radiation-less processes in which it will only release heat. It can give radiations or it can undergo chemical reactions. So, it is not always necessary that an excited molecule will undergo a chemical reaction only. The fourth option which is - light is essential for a photochemical reaction. This is true as we have studied that light is the basic need for providing energy.
Thus, we have the correct answer as option a.), option c.) and option d.).
Note : Photochemical reactions can-not occur without the presence of light. Further, an excited molecule may come down in ground state by losing energy in the form of radiations like fluorescence and phosphorescence. The radiation-less transition by losing energy in the form of heat is also possible.