Run bitbucket-pipelines.yml file on pull request merge to master

[Vishal_infy]

I know that there is a possibility to restrict the bitbucket-pipelines.yml to run on specific branch using the following syntax:

image: node:4.6.0
   pipelines:
   default:
   branches:
     master:
       - step:
           script:
             - echo Hi
             - echo How are you

The above syntax would run the yml pipeline on commit to master branch.

But this there any way to run the pipeline only when the "pull request is merged to master branch" but not when a commit is done on master branch?