The spectrum of ammonia in gas phase consists of one broad electronic absorption band  at λmax 179. What is an extinction coefficient of ammonia at 212-215 nm?

I would use energy considerations to estimate the dissociation threshold:

delta_Hf(NH3)+hv = delta_Hf(products) where deltaHf are the enthalpies of formation of the corresponding elements which can be found in NIST Chemistry Webbook.

One has to remember that even if the products react with heat production, the first threshold in the chain will determine the dissociation. Most probably, lambda - 179nm = 6.93 eV is the threshold the author should be looking for since the enthalpy of formation of N (one has to consider single atom even though a molecule can form later) is high and is equal to 472kJ/mol (almost 5eV).

Three parts of this question are:

[A]Whether 215 will decompose NH3(g)?

1 kJ/mol = 1.1963*10^5nm

So 215nm= 1.1.1963.10^5/215=556.42 kJ/mol

Now heat of formation of NH3(g)[ from lilerature]= -46.1 kJ/mol

So heat of dissociation of NH3(g) [Reverse of formation]= 46.1 kJ/mol

Again 215 nm which falls in UV region can decompose NH3(g)

So the answer to this part is YES.This part ends here.

[B] Power required to decompose one liter of NH3(g)

46.1 kJ value is for 22.4 L of NH3(g)

Heat required/ L= 46.1/22.4=2.058 kJ

Convert kJ into into kW

Use the converter below [Reported in the literature].

1 Kilojoule = 0.000 277 777 778 Kilowatt Hour (Kwh)

Kilojoule is metric system energy unit (electrical,mechanical,thermal) and equals to 1000 times of amount of work done by a force of one newton to move an object one meter.

Kilowatt hours is one kilowatt amount of power delivered in one hour. One of the most common usage is billing the amount of energy delievered by electric utilities. The abbreviation is "kWh".

So 1 Kilojoule = 0. 277 777 778 watt Hour

And 2.058 kJ = 0.571667 watt hour

So you need 0.571667 watt to decompose one L of NH3 in one hour .But you want to decompose one L of NH3 in one minute, supply0.571667.60= 34.30 watt in one minute.

In a way , it is = 0.571667 watt hour= 34.30 watt in one minute

[C] Conditions to increase rate of decomposition of NH3.

As it is a photochemical reaction, it is not expectected to be affected by temperature/ pressure [Lechatelier’s principle is not applied]. . Had it been a thermal reaction,we would have applied Lechatelier’s principleand there by it would have been affected by temperature/ pressure

Again 215 nm energy is more than sufficient to decompose it.

Note: May I humbly suggest you to READ THE ATTACHED FIL WHICH MAY ENLIGHTEN YOU MORE ON PHOTODEMPOSITION OF NH3(g)[Zoom200-250].

and where are the enthalpies of formation of products in [A]? Following [A] in its current form, H2 dissociation doesn't require any energy (delta_Hf(H2)=0) while in reality delta_Hf(H2)+hv = 2 delta_Hf(H)= 2*218 kJ/mol = 436 kJ/mol.

[A] If heat of formation of NH3(g) is -46.1 kJ {reported}, then its heat of dissociation is +46.1 kJ; heat required to dissociate 22.4 titres of NH3 (g) at NTP.

[B] And above all, we all follow simple Hess’s Law of constant heat summation- which IMPLIES if 46.1 kJ is stored in one mole of NH3(g) during its formation, the same amount of the heat will be reqired to dissociate it.

[C] And Still and other point, we are neither dissociating H2(g) norN2(g). We dissociate  NH3(g)  only into H2(g)[ not into two H(g)] and N2(g)[ not two N(g)].

[D]Finally, without further contesting, I leave it to my young celebrated friends and would, humbly, accept their arguments.Perhaps, we old people belong to an old school of thought viz.a.viz. thermochemistry.Thks.

I'm an old person belonging to the old school of thermodynamics and kinetics.  You must consider how many photons are consumed for overall process 2 NH3 -> N2 + 3 H2.  The statement   "If 46.1 kJ is stored in one mole of NH3(g) during its formation, the same amount of the heat will be reqired to dissociate it" is absolutely wrong. The dissociation energy H2N-H bond is 435 kJ/mol. The energy of ONE 215 nm photons is equivalent of 556 kJ/mol, which is enough to dissociate one N-H bond.: H2N-H -> NH2 + H. After this primary process the kinetics becomes involved.  Three reactions are possible: 2 H -> H2 , 2 NH2 -> N2H4., and H + NH2 -> NH3. Roughly, only a half of formed H  will produces H2. Thus you will need FOUR  photons two generate H2 and N2H4. After that you need to convert 2 N2H4 into N2 and 2 H2. Additionally you will need at least FOUR more photons, totally more than 8! To make 8 photons you need 8 x 556 = 4450 kJ/mol. The rate "one liter of ammonia in one minute" is ~ 7E-4 M/s, which requires a power (4.45e+6)*(7e-4) = 3 kW. This is a total waste of energy. 

I just applied the logic from the first  [A] statement to the case of H2 dissociation to illustrate that this approach is not correct while a "standard" one (enthalpy of formation in the left-hand side plus energy of the photon equals a sum of enthalpies of formation of the products in the right-hand side) immediately gives the energy of the H-H bond.

The statement "If 46.1 kJ is stored in one mole of NH3(g) during its formation, the same amount of the heat will be required to dissociate it" is absolutely wrong .My God!.Hess’s Law no longer holds good .Thks.I know it only today.And will submit before your argument.

Dear Mahomar. Please use well defined terms in your statements. "to dissociate it" means in the discussed reaction the photoinduced dissociation of H2N-H bond, This requires energy of 435 kJ/mol. The Hess's Law is thermodynamically correct. The reaction 2 NH3 -> N2 + 3 H2  is NOT the dissociation! According to IUPAC 

The dissociation is 

1. The separation of a molecular entity into two or more molecular entities (or any similar separation within a polyatomic molecular entity). Examples include unimolecular heterolysis and homolysis, and the separation of the constituents of an ion pair into free ions.

2. The separation of the constituents of any aggregate of molecular entities.

In both senses dissociation is the reverse of association.

That was the reason that I used “This part ends here.” [ Photochemical aspect] at the end of [A] .These words went unnoticed. Any way, we should stop and give our young friends a chance to contribute. Rgds.