Cook artichoke hearts according to pkg instructions, then drain well and finely chop. Cook bell pepper in 1 tbsp butter in a 2 to 3 quart heavy saucepan over moderately low heat, stirring occasionally,until softened, about 5 minutes. Stir in artichokes and transfer mixture to a bowl.
Melt remaining 2 tbsp butter in saucepan over moderately low heat, then add flour and cook roux, stirring, 3 minutes. Add half and half in a stream,whisking,and bring to a boil,whisking. Reduce heat and simmer,whisking for 3 minutes.
Remove from heat and stir in artichoke mixture,scallions,1/3 cup parmesan,lemon juice,jalapenos,salt, and celery salt. Gently stir in crab. Transfer to a buttered 1 1/2 quart shallow baking dish and sprinkle with remaining parmesan.
Bake dip in middle of oven until bubbling, 20 to 25 minutes. Serve warm.
*Dip can be prepared (but not baked) 1 day ahead and chilled,covered*